How long did you leave on? It often needs to be overnight. And make sure it is on tight enough to keep the power button pressed. You may have done all that but wasn't clear with the details you provided.
Hi. This warning is very important and is regarding many phones in fact. Your provider has to populate your phone with IMS files on first try as i know. If you don't verify if VoWifi and VoLTE are working fine from stock, you could not get this working well after flashing a new rom. That's already happened to me ☹️ many times.