Thank you Martin. Deep down I really think I’m never going to get ARC running on my current router so I will make enquires with Spark to see if they can supply a different, newer, router or port forwarding advice.
This is a bit of a pain as the current router works flawlessly with everything else I throw at it
That’s the right one and it should allow Roon to open the port - just like it seems to be doing for Plex. Very odd
If that’s the public ip you got from your provider?
It doesn’t matter whether it’s static or dynamic.
But you should definitely not post a public ip here, better delete it!
Thanks! I’ve deleted that post. I’m just going out now but will repost when I get back.
New Zealand … have a nice sunday!
I’m going to sleep now, it’s 12:30 p.m here 
Try
You need a public IPv4 adress for checking canyouseeme.org above.
And if canyouseeme.org can’t see your router’s port, ARC can’t see it either.
OK, to repost my previous deleted post.
This is interesting. I tried the common ports plus Plex 32400 and Roon 55000 in canyouseeme.org and get this error message:
I can confirm that the 222.152.. IP is correct as shown in Whatismyip.com
Next I entered FTP in the Port Mapping page of the router, then tried port 21 (FTP) in canyouseeme.org and still get the error message.
I’m at a loss now. 
Update: I opened Plex Server on the MacMini, went to the network settings and there it states:
Public 222.152.. 17925
I then tried that in CanYouSeeMe.org and it worked.
I then entered 17925 into the ARC settings page (with both Plex running and not) but still no joy.
Sorry Mike, i’m out of ideas either. Except for such mundane ideas as power down and rebooting the entire network, including your phone after uninstalling ARC and then reinstalling it. But you probably already did that anyway.
It’s strange and I’m sorry that I can’t help you further.
Could some firewall possibly play a role? This does not necessarily have to be on your side, it could also be on your provider’s side, in the form of some kind of filtering.
I think that’s good advice and the only thing left for you to do.
Thanks so much for your time Axel. Yes, I think we’ve exhausted every option - I’ve rebooted everything multiple times and gone through every option I can think of on the router. I’ve spent way too long trying to get this to work so am officially giving up.
Have a good Sunday! 
What screen do you see when you click on + New port mapping?
This is the screen you see when you click the “New Port Mapping” box
There is nowhere to manually add a port number number
Yes, it does look like all options are exhausted unless your ISP can assist.
Good thinking. I’m also at a loss though. I assume that the Room ARC settings claimed that all is good after you entered 17925?
I’m wondering too. But then I would expect that it affects Plex as well.
Thanks Martin, I agree.
No, when I put 17925 into ARC settings it still wouldn’t connect and gave me the same error message.
Certainly from my end there aren’t any firewall settings that would block ports - and like you say, Plex works perfectly.
