I tried rebooting the phone a lot, and the core a little. Neither helped. But now I rebooted the core AND the phone, and lo and behold, it works now. Many thanks! Oh man, that has cost me a few hours LOL
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I am an idiot. I was on wifi after the phone reboot. No, unfortunately no change.
well, thatās too bad. 
Aha, I can use telnet on the laptop to connect to the coreās external IP:port when I go through my cable internet, but when I connect the laptop to the phone hotspot, I canāt.
You may want to check against actual roon core ip or use roon settings->roon arc to test.
I did both. The ARC tab in the remote settings says that all is fine. So does an external port checker website, when entering the coreās external IP and port to check - it answers. And I did telnet from the laptop to the actual external roon core IP (and port). It connects just fine when my laptop is using the cable internet. However, it fails to connect if the laptop is using the phone as a hotspot. Something seems off on the phone or whatever it is Vodafone is doing in the phone network
Very interesting, if you cannot figure out what is going on then it must be something out of ordinary.
I made progress. Thatās another half Saturday wasted Just came home from an ARCing trip outside So the thing is this:
- The IP address and forwarded port works. I can connect with ARC just fine from the wifi in the corner cafe. (So all the info in the ARC tab of the remote, and the telnet test via cable internet, and port scans via websites, etc., did show the correct result - the Core and its internet connection are OK)
- However, no dice over Vodafone Germanyās phone network. Drove the bike a bit to also ensure I am in a different cell tower area. All the same, I canāt reach the coreās external IP from the phone network. (So the telnet test when being tethered to the phoneās internet also showed the correct result - fail). I also tried with a different SIM from a different contract (but also VF) in a different phone, same issue.
Next step: Vodafone technical support. I already talked to them in the morning and was lucky to have a lady who understood what I was talking about. She flipped some switches in the settings for my SIM, but that didnāt help yet, and for this case she gave me a way to contact them back, so I donāt have to tell them the whole story again. And armed with my new results, I think we have zeroed in on where the problem is.
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Well, how disappointing. I guess I will not be able to take advantage of ARC. I am an audiophile, but not an IT professional. The solutions offered to make the ARC connection work are way beyond my understanding and technical prowess. You might as well write the instructions in Greek. I donāt know Greek, but I might have a better chance getting ARC to work with that.
It seems to me that roon owes a solution to those of us to whom port forwarding and related protocols are beyond our comprehension. Certainly there is a way to get ARC to run without having to be required to hire an IT professional to make it so. Iām willing to bet that I am not the only roon customer deep in the swamp with this.
If my only option to use ARC is to download files in advance to play them, why the heck would I so that? Iāll just take along my DAP with about 600gb of storage and enjoy hassle-free sound that is far better than what my phone could provideā¦
Very disappointing.
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I sympathize with the comments of Henry_Grillo. I cannot use ARC on a consistent basis outside of my home. Just recently, I open ARC and Roon at my residence using Wifi. I can get Roon ARC to work using wifi and have adjusted the port settings above. However, as soon as I am on a T-mobile network within the DC metro area, I cannot hold a connection. I have enough bandwidth from the cell provider as Apple Music, Qobuz, Spotify work without any failure. I continue to receive Poor Connection or Roon cannot Connect messages. It is a disappointment that ARC will not allow me to connect to my library.
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I guess the question is that the cell providers work with the main streaming services but something in ARC is not allowing streaming/connection. If it is the port forward, there needs to be a specific user manual for the layman, walking through the steps one by one.
Ok, please whoever is having issues with connecting outside of your network and getting Poor Connection or Roon cannot connect, try this:
- On iOS, go to your Roon ARC under Settings.
- Select Roon ARC App and check your settings.
- Select Sir & Search and DESELECT (turn off) Suggestions
a. Show on Home Screen
b. Suggest App
I may have turned off āLearn from this Appā as well. Please confirm if this works for you. I tried with my wifi off on my device, and it appears that my connection to Roon works. I have troubleshooted here at my residence with the wifi off on my device and been hoping around in my library and have had fast responses thus far and the music had played! I will try again when I am on the road and outside of my residence. Good luck!
Here are my screen shots, hopefully the order makes sense.
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Sadly, that did not work for my setup. āCannot connect to your Roon Core.ā It does work when I turn WiFi back on, for what thatās worth.
Did you turn off your router or did you turn off WiFi on your phone? I turned of the phoneās WiFi and left the router WiFi on.
Thanks for the update. I also was not able to connect to Roon Arc as soon as I left my residence. At this point, I do not believe there is a solution to use Roon ARC over the cellular network with T Mobile in the DC metro area. I will continue to use the Roon app while I am home and stream Apple Music, Qobuz, or Spotify while I am away from my residence.
Is there a way to reach Roon Support to open a ticket on this issue? I am still having the initial issue.
This is the open ticket, so to speak. Roon does support here in the support area of the forum. Itās just that their queue is currently long and it often tends to take them about 10 days as far as I can tell from other threads. Of course you have been waiting for 21 days 
Maybe @support can take a look please?
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Hi all-
Iād like to share what I did for Room ARC to finally work when Iām away from my home. I went into the admin setting for my router (Fios) and made my ROCK (an Intel NUC i7) a DMZ IPv4 Host by adding the deviceās IP to DMZ:
DMZ Host
Apply Changes
How it worksā¦
Allow a single computer or device to be fully exposed to the Internet.
DMZ IPv4 Host
Enabled
Local Host [IP address]
It seems to work now as my ROCK is fully open.
You shouldnāt do that
Ah ok. I see. Thanks @Suedkiez Iāll disable DMZ. But there seems to no solution, right? I have to open up a ticket for a support staff that is already inundated with requests for the same issue. Very frustrating. Is there another alternative that you know of?
Generally with public IP issues, many people reported success with Tailscale or other VPN solutions:
https://community.roonlabs.com/search?context=topic&context_id=216404&q=tailscale&skip_context=true
But the solution depends on what the cause of the problem is. I donāt understand the problem described in this thread, so donāt know if that works