I am able to do a Portal Login on PC if I do the Authentication Challenge procedure, which is a hassle.
Interestingly, after I tried first to totally quit the IBKR app (not just exit it) on my iPhone (removed app from memory, need to first double tap the iPhone "home" button, then swipe away IBKR). IBKR then when I tried the PC Portal Login normally started up and did the normal easy authentication. However later Portal Logins did not work. Maybe it is a bug in the IBKR app,
I am installing latest iPhone iOS 14.3 and will try that later, in case that fixes it. Also, it might be a firewall problem, or maybe something related to my cellphone wireless service company, IB notes say the Portal Login with phone authentication might fail if there is something wrong with the wireless connection. I normally have cellular data turned off and use Wi-Fi data on my phone, maybe that's a problem.
I will try contacting IB if problem persists after the phone iOS update.