Not true. I just connected my iPhone and two iPads to Roon ARC, all using cellular.
EDIT: And now, over WIFI.
Yes @jim_f 
After I said this I tested myself and posted to say I was incorrect.
Not a 100% sure why the OPs situation switched around. May just be an unwanted coincidence.
Ok, I think I’ve found out what was wrong. I’ve been through everything on my home network and checked every device; hence the long delay since my last post.
What I found was that Roon ARC was reporting that the server is setup to use Port 55002. To get Port Forwarding to work on our BT Smart Hub 2 you have to manually create a ‘port forwarding rule’ in the advanced firewall menu. I had done this and I’m convinced that, when I first created the rule some months ago, the port address was set to 55002. And Roon ARC worked when I was out and about. But now the port address in the hub is set to something completely different. I’ve no idea how that happened. Difficult to see how it could happen accidentally as you have to log on using the admin password and then its 3 layers down in the menu. I cannot believe that the cat walking on the keyboard could have managed that!
Anyway, I’ve reset the port forwarding to 55002 and now both my iPhone and iPad run Roon ARC away from the house, even simultaneously playing different tracks 
Thanks for all your inputs; you convinced me that Roon ARC really does work and that therefore it had to be something to do with my configuration. The confusion was down to Roon ARC reporting that it was setup properly whereas in fact the router was not. How I was getting either device to do anything with Roon ARC away from the house is a complete mystery.
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