I installed Roon ARC on my iPhone(14 pro). During the login process, login was completed in the Safari window, but at the end, login failed with the message 'Something went wrong Please try again’. However, I installed Roon ARC on my iPad(pro) and logged in successfully. And currently, the message 'Syncing with Roon Server for the first time...' is displayed.
Thank you for taking the time to write in, and welcome to the community!
Taking a look at your iPhone issue first, are you able to test using a different web browser on the device?
Does the same issue occur over cellular data rather than Wifi?
If you haven’t yet, I’d also safely stop Roon Server from running, and power down the device along with your router. Then reboot the two and let me know how things perform.