Earlier this year Roon ARC was working fine on both my iPhone and iPad. I just tried it again as we are going away for the weekend and I got exactly the same symptoms identified here ARC app issue with Roon Server despite secure access indication (ref#H24F23). Naturally I am not aware of anything changing in the interim.
The good news is that uninstalling and re-installing Roon ARC on both my iPhone and iPad fixed the problem.
The bad news is that a year on and the problem is still occurring!
Hi Adrian,
I had opened the thread you were linking to and I just can say that this still happens to me every now and then. It’s embarrassing that the only option is to reinstall ARC then. I have stopped downloading selected albums in the meantime and just let the Smart Album Download do it’s job when it is working again. I don’t want to spend time again and again for something that doesn’t last.
In the end this has lead to a mindset that ARC is “nice to have” when it works as expected but nothing I would rely on. This is quite sad given how good ARC would be when it was working reliable.
I have had the same problem but have been finding that downloading albums and listening to them off line only may be the answer
Yes, I guess that ARC is a bit niche compared to the main ROON functionality so as it now mostly works, any fixes are lower priority. 
I had similar problems and tried a workaround which also made my network much safer. Instead of using a port-forwarding as recommended I installed a Wireguard VPN in my router and gave access to my iPhone. When leaving the house I activate the VPN and then use ARC. Much more stable and reliable plus I could remove the port-forwarding. Give it a try, it also works pretty well in my car.
Thanks for the tip Manfred. Unfortunately I don’t believe that our BT Home Hub 2 router supports that kind of thing. It’s very reliable and stable but does not allow any fancy messing with it. Which is probably why it is so stable!