Hi @wally , thanks for your welcome.
That is right, that is my locality. Hmm, the picture darkens for my chance of using the device (and, really, Ubuntu Touch in general) then. I had already made peace with the fact that the "5G" in the device's name would not be reflected in its performance. But, I assumed with the phone on AT&T's supported devices list, it would use either 4G (though from what you're saying it sounds like LTE is the only version of 4G available here) or I didn't even think about 3G. I guess that their list was just if you stick with iOS or Android.
This particularly complicates my decision, not because of needing to use this particular phone (luckily, I only sunk $100 in buying this used model), but because I'm not aware of any other options that meet my needs here in the U.S. anymore. The only other phone I noticed on both the Ubuntu Touch-supported list and the AT&T-supported list that was even a mid-range model was the Pixel 3a. All phones made to explicitly work with Ubuntu Touch seemed to be only available in Europe (where I understand there are more people up for a pro-privacy tech mission).
I appreciate your input on call (lack of) possibility, so I don't waste my time on a futile task. I also appreciate your suggestions for how to work with the MSM issue I've had, though I don't think I'll try again if I can't actually use the phone as a phone.
I guess my next plan is to look into the feasibility of GrapheneOS, CalyxOS, etc. However, this was definitely my less-preferred plan, as I think it's naive to think that de-Googling a Google OS is fully fighting the behemoth; if you're using Android of any form, as far as I can tell you're buying into an open-source OS that really is there to serve the tyrant, and which they can have (and seem to be gradually doing) a tightened, more-closed process.
Therefore, if you @wally or anyone else knows of a way I can get Ubuntu Touch on a call-able phone in the U.S., I would appreciate your suggestions (I was looking to at least have a mid-range few generations old phone, but at this point I'd consider lower than that). Otherwise, I just want to say I support what you're doing, and hope the Ubuntu Touch mission can continue to expand and grow across the world in the future.