Hi @qobuz_M, welcome to the community. Roon ARC login issues on iPhone usually come down to either account access or the phone not reaching your Roon Server over the network.
To get a clearer picture, a few things would help:
What happens when you try to sign in to ARC on the iPhone, does it fail on the login screen, or does it sign in and then say it cannot reach your Roon Server?
What device is your Roon Server running on, and is it connected to your router with Ethernet or over WiFi?
Are you trying this from inside your home network, or only while away from home?
If you have not already, it would also help to confirm that your Roon Server is running and that the phone has a stable internet connection. If the phone is on your home WiFi, ARC should still be able to reach the server through your network setup, so the exact error message will matter here.
Reply with those details and we can narrow this down from there.