You normally shouldn’t have to do anything at all beyond inserting the SIM.
Did you test it under Android on the same phone?
What’s the band and coverage situation like?
This won’t help, as you noticed.
These settings come into play only after having found the network, so that is reasonable.
So maybe the automatic connection “works” but runs in to this same problem.
Are you using some picky (US?) operator that applies arbitrary whitelists, or that demands VoLTE?
Probably allowed/preferred by SIM.
Is your new operator a MVNO of the former?
Have you tried using any of the older technologies (2G/3G)?