Is part of what I meant by
We seem to be working along the same lines … get physical fully working first …
There I refer to the things that have already been thrown at the device.
If necessary we can explore the “presets” which come with the /e/OS install if you find any evidence that your carrier’s published settings for your plan / SIM are wrong, like this Wrong APN settings Telia Norway.
- When you (earlier, Post #2, 3)
Reset Default + Airplne and
Reset Default + restart you hope to connect to your carrier, and the carrier automatically reponds by trying to write the correct data wherever needed to do the mentioned “harmonisation”. - You would get your carrier’s published APN and create (add) a new APN set at
Settings > 
Search: Access Point Names.
So loosely speaking IMS is a one off event where the carrier registers the device “permanently” into their network specifically for
The system carriers use to deliver modern voice and messaging over 4G/5G. If “IMS registration” is shown as “Registered”, your device can use VoLTE, SMS over IMS and Wi‑Fi Calling (subject to your carrier plan and device support).
So we used to be able to live without it … increasingly it is required for advanced methods which carriers may use under the hood and maybe especially in virtual networks (you mentioned MVNO Mobile virtual network operator).
Understood, but “Divide and rule”. I kept my e-SIM comments on a separate post just so we understand what we are trying to do. Two problems will be interdependent, acknowledged) but my training says never attempt to solve two problems at the same time or you will never know what fixed what.