the ARC page in roon says my server is confirmed to be securely accessible to Arc. I start Arc on my iphone and it does an initial download, ask who's listening. I choose the only option (myself) and I get "There is an issue with your roon server" and tells me to go to the roon app for more information. I'm already in the roon app so that is not helpful. My only option is to disconnect from the host running roon server. I do that and I'm back at the beginning.
Can you temporarily get a direct ethernet connection from your Windows running Roon Server, directly to your primary router? Let me know if you continue to run into Arc issues after this - it may be worth giving your router a reboot as well.
If you do still have issues, are you able to share the amount of free space on your iPhone you’re attempting to use Arc with?
If you haven’t yet, I would remove Arc and perform a fresh install over stable Wifi as well.