[Upgrading, Modifying and Unlocking] droidzer1:
If you have a phone which needs APN settings modified for it to function fully this may be of interest. In many cases you can simply go into Settings > Network > Network and Internet > Mobile Data and add an APN or edit an existing APN.

However in some cases you may find as I did that the SIM / phone combination does not allow you to edit or add APN settings. If you have root their may be ways to edit the APN configuration files directly. However with Android 10 and some other situations that becomes difficult if not impossible. In my case even the phone network service support could not update the APN setting. I knew what the APN settings needed to be but since I had an unrooted Android 10 device (a smartwatch in this case but it will work for phones also) I could not get access to folders to edit the configuration and it is unlikely it would have been saved due to Android 10's read only system partition.

On this device I had cellular phone calls and text working but NO Mobile data. I could see the APN settings were not correct for the service I was on for Mobile data to work. I tried everything for days to edit but it was grayed out and I could not add a new APN. In the process of pursuing a solution I found that putting in a different SIM (in this case a Textnow SIM for their new free phone service through Sprint) that I could edit the APN settings when that SIM was inserted. So you will need to find a SIM card that will allow you to edit settings.

Now the real trick involves hot swapping SIM cards so if you aren't comfortable with that then you won't be able to do this. There seems to be differing opinions on this but no phones or watches or SIM cards were damaged when I tried this.

What I did was edit the other SIM by changing the APN settings. Then when I hit the back key it said something to the effect the settings were not allowed on this. At that point I pulled out the SIM with the device ON. I then put back in the SIM for my service provider with the device still on. The settings showed up now on my formerly uneditable SIM ! Mobile data now worked fine. I

I had a second device I wanted to put the SIM in (identical to the first device) hoping the settings would still be there but they were not. Apparently communication between the SIM card and the phone gets APN settings transferred to the phone or watch. So I wanted to do the same procedure on this second device. In attempting that it did not seem to be working the same at first. However after trying for about an hour it finally worked on the second device also. So persistence pays off if it doesn't work at first. I kept turning things on and off in the Mobile data settings, rebooting and playing around with developer options. I wish I could say exactly what got it to work on the second device or why it just worked on the first device the first try but I haven't been able to pin down the variable which achieved success.

But the conclusion is that is you can't edit your APN settings normally you may have success with this method if you keep at it a bit. Good luck and please post here if you have success with this method and share any insights into how you got it to work for your device.