To clarify this issue:
Under Settings > Network & Internet > Mobile Network > Advanced > Access Point Names
In a screen entitled “APNs”, it reads “Access Point Name settings are not available for this user”.
At the top of the screen, you’ll see a “+”. Click that, then find the APN settings for your carrier:
https://www.att.com/support/article/wireless/KM1062162/
https://www.t-mobile.com/support/devices/not-sold-by-t-mobile/byod-t-mobile-data-and-apn-settings
And enter in the correct data in that screen.
At the top-right of the screen, there’s a ‘3-vertical dots’ menu, after entering in all the data, tap that and select ‘Save’.
Back out of that window to the main screen, then click on the “Access Point Names” to take you back to the APNs screen you were just on.
You’ll see your carrier (ie: the data you just entered), with a circle that you need to tap on to select it.
Now you can exit out of everything and reboot your phone.
As a potentially-related aside, could this be why App Lounge has to have Mobile Data turned on in order to work? Because now my App Lounge doesn’t crash out when I access ‘Updates’ with Mobile Data turned off, it instead says:
Timeout fetching applications!
Some network issue is preventing fetching all applications.
Open settings to look for Open source apps or PWAs only.
OPEN SETTINGS RETRY
Turning on Mobile Data gets the same thing, but at least now App Lounge isn’t crashing.
Turning on Mobile Data shows “H+”. It never showed that before. That’s Enhanced HSPA… providing up to 168 Mbit/sec downlink speed. That’s one step below 4G. I likely get that because I’ve not got a strong signal. Before I manually set up the APN settings, I was only getting 3G at this location, and only about 300 Kbit/sec.