This is a super long story, but I'll try not to go on too long.

My Pixel 3a running Ubuntu Touch 24.04 (24.04-1.3) abruptly stopped being able to send/receive texts/calls a few weeks ago. And it was unable to connect to my carrier's network.

I took it to my carrier and got a new SIM card, after which it was able to connect to the network (it shows my carrier's name in the "network" section on the swipe-down-from-top menu), but it still can't send/receive texts/calls. (Also, a borrowed SIM card for a different carrier -- but which happens to use the same network of towers as my current carrier -- didn't work when inserted into my phone.)

Mostly just for troubleshooting reasons, I'm looking to return to the stock firmware. (Given that it worked fine and abruptly stopped, I'm guessing calls/texts won't work on the stock firmware, but that'll at least be pretty solid confirmation that it's a hardware issue. If somehow it does work on stock, I'll probably try returning to Ubuntu Touch and see what happens.)

I've got developer mode enabled on my phone, and I've got USB in "File Transfer" mode. But adb reboot bootloader gives me Failed to write reboot parameter file: Permission denied. I think this is an error from the phone, right? Not an "ADB can't connect to the phone" error but more of an "ADB told the phone to reboot into the bootloader, but the phone responded with this message" sort of error.

Am I wrong to assume that adb reboot bootloader should work with my phone when it's running Ubuntu Touch?

That's basically my question. Thanks very much in advance for any assistance you can provide!

Edit: Oh, forgot to mention the Google "Android Flash Tool" that runs in the Chrome browser doesn't have any better luck than ADB.