Is a known issue … I believe it is important to have a good Wi-Fi connection while doing this … carriers will vary, but at least some will attempt to verify (over Wi-Fi) that you are setting their valid APN.
How hopeless … in the case of no connection it is up to them to send you the “special” text that will include APN reset.
These days I think there is some Google magic involved to aid quick resolution of APN … it can take some users a bit of patience as seen in other apn threads.
Of course the 5G might compound things … just for clarity … this was an old SIM:
… was the aim to get a new 5G capable SIM and this is the start of the problem?
Any clues in this thread [LIST] Carriers that work or do not work with /e/OS country-wise maybe if you share the carrier name someone using that service will chip in.