Well, I gave zerotier one it a try and summarize my findings below.
Created a PUBLIC network in my.zerotier.com. No additional configuration there.
Installed zerotier one on Win10 PC (I call ist “mediaserver”) located in my home network behind a NAT router. This PC also runs the Roon Server.
Installed zerotier one on my Win10 Notebook that is connected via LTE to a public network.
Made no custom settings on these two PCs.
After joining my zerotier network, both PCs show up. From my Win10 notebook I could ping the mediaserver.
When I start Roon on the Notebook I can conncet to the roon core on the mediaserver after inserting the IP zerotier one gave to the mediaserver.
The audiointerfaces of the Notebook show up and I can play music via the notebook from my roon core at home. Anything Fine 
Next I installed the zerotier app on my iPad, that is also connected via LTE to a public network. From here I also first tried to ping the mediaserver worked! Then I started Roon on the iPad. It finds the Roon core after submitting its zerotier one IP, same as with the Notebook.
But there is no playback device available for the iPad so I cannot play music on it. I can see the audio devices of the notebook and of the roon core but not the output device of my iPad 
To find out why I enabled a WLAN access point on my mediaserver. This zone has a different IP as my LAN at home.
Connecting my iPad to this access point via WLAN and starting Roon I can see the roon core (after submitting its WLAN IP) and i CAN see the output device of my iPad an play music with the iPad.
To summarize it up:
My impression is, that the iOS implementation of zerotier one does not transmit the UDP broadcasts to the roon App in the iPad. I have no idea how to make that work.
As I have noted ZT doesn’t work for the iOS apps but I think Android might work
I asked the support of ZT and got this response:
iOS and Android give us no way to enable multicast on the network adapters we receive from the OS. Unfortunately there’s nothing we can do about that.
This seems to be the reason why playback on an iOS device connected via ZT is impossible.
Now we know…
This is a mistake. You’ve opened yourself up to anyone that determines (e.g., via “wardialing”) your network number. Please use a private ZeroTier network.
Thanks for the hint. Started with Public switched to Private 
Hi wizardofoz,
I have been trying to get VPN working with iphone with not much luck but kept trying as people in the forums say they can get it working.
I’ve tried the VPN in my router, now I was thinkering with zerotier and saw your comment:
“As I have noted ZT doesn’t work for the iOS apps…”
So this seems like it can’t work…
I am confused now!
It vpn from iphone possible? it seems not, but how come some say they have it working…
I haven’t found a way for iOS but it’s not high up my priority list right now I can barely leave my home for another 6 weeks
Hello
But how us your setting.
I installed zero tier 1 on my android phone. Appear online bu if i open roon in my android phone appear the warning that its necessary a wifi.
The computer where roon is installed must be always active?
I installed zero tier in my laptop where roon core and software is installed.
Please advise
Thanks
You need to have a wifi connection…either hotspot might work ok but a proper wifi connection seems to be no issue…same for iOS/iPadOS
In fact, you don’t really need wifi ON on your Android, mine connects through LTE just fine. It’ll say it needs a wifi connection, but in a couple of seconds the Android on LTE would see my Roon Server. One thing to pay attention to is to make sure your Roon Server is on “private network” not “public network” when checking “network status” on your Windows machine where Roon Server is installed.
My Android has no mobile option as its a DAP but with wifi connections its pretty fast. Tho I haven’t used it in a long time I can’t remember if I had to put an IP address in or not now.
Hello
I installed in my Windows 10 the zerotier program.it is in “private” mode, but how can I connect it to my android? I also installed zero tier in my android and entered the number that appear in the zero tier, see screenshot
Another point to check might be to go to my.zerotier.com/network, to makes sure both your Roon Server device and Android device are checked underneath the “Auth?” column? I am not a network guru, just a regular user here.
Hi,
I am heavily using ZT in my home and office network. I am quite sure that if ROON CORE is on a Windows OS or Linux OS with ZT installed will work like a charm for remote access.
But the problem is I am using ROCK as a ROON server at home, and I installed ZT in my router system. Is there anyway for getting remote access possible in such setup?
Thanks a lot.
Rock cannot run anything other than rock it’s a closed operating system running as an appliance.
This will not work. The reason ZeroTier works with Roon is it creates a new virtual Ethernet interface on the Roon Core system and extends that virtual LAN segment to the ZT instance running on the client. While ZeroTier can be configured as a routing gateway, this no longer presents itself as a directly connected (SD-)LAN segment and Roon Core does not (without much trickery) support routing to endpoints. Roon Core expects all endpoints to be on a local, directly connected LAN.
Since ROCK is a closed “appliance”, you cannot install ZeroTier on it. If you want to do this, ditch ROCK, install your favorite Linux distribution and then install Roon Server manually. (FWIW, my Roon Core runs on a CentOS 7-based installation, where I have also installed ZeroTier.)
I am not sure if anyone is able to help me at all?
I’m not really after using Roon outside of my home, but I do have an extensive library of music at work that I would like Roon to be able to access!
My Work PC is Windows 10,
My Home PC in an iMac Big Sir (this also has my Roon running on it)
I have used zero tier, and I am able to access my iMac from Work, and also access my work Pc on my iMac at home. I can mount the drive I want to access on Mac (it shows SMB) but when I try to add is as a network share in Roon it just shows “Unauthorised”
Im using the same detail that I use to access it on my iMac, so does anyone know why I can’t get Roon to see it?
Any help would greatly appreciated!!
You should maybe reread and edit your post as it’s seems you can or can’t do some things that you seem to want to do but maybe just worded it badly or in the negative when you meant positive or vice versa.
Adding a network share is no different if ZT can provide the connect. If macOS can connect the share roon “should” be able to also.
Bless you, thank you much! I’m not so good with technology! 
I thought it should work too, certainly as my iMac can connect and mount the drive.
So I am really unsure why Roon can’t add it as a network drive…
I click onto add network drive in Roon, enter all the same details I use to mount it on my iMac and it just says “unauthorised”