[Windows 10 Mobile] Alec.McT:
So tantalizingly close to full functionality of Verizon Lumia 735 on T-Mobile network, but after a blissful full day of trouble-free operation, back to phone not connecting to LTE, with connection to lower-speed signal cycling on and off.

Timeline:

On July 23rd, had phone working well after upgrade from WP8.1 to 10.0.14393.1480. While travelling in Canada (home service is in US) phone continued to work well. Until I connected to hotel WiFi on 8/8, and - thank you automatic updates, and why can't I find a way to make hotel WiFi be seen as a metered connection?? - received an update to 1593. But that update didn't successfully complete due to me not being connected to WiFi when system restarted later in the day. Phone stopped working reliably at this point.

At home: fresh install of 8.1 via Windows Device Recovery Tool - phone functioned well, picking up LTE wherever present. Ran that for a few days without problems.

Upgrade (via regular channel - upgrade advisor app, phone update settings panel). After all updates successfully completed, phone was again running 10.0.14393.1593. Bad behavior immediately after 1593 update, but one more power off/on restored normal operation. I had a full day of excellent operation with this setup - in fact, stronger / more consistent LTE than I had been getting with 8.1.

...and then... nothing. Phone just stopped picking up LTE, is mostly without signal, and no amount of power cycling or soft resets kicks things back in line.

I've tried:

Added MMS APN as I had done in the past here. Didn't expect that to have an effect, and it didn't. But I had that change in place last time my phone was working, so what the heck.

Showed and then selected "4G only" as highest connection speed in SIM settings. (To make option available, used Interop Tools to add registry key HKLM\Software\Microsoft\Cellular\MVSettings\IMSISpecific\Default\CellUX\HideHighestSpeed4GOnly = dword:0). With this setting in place, network search shows multiple LTE networks available. But selecting mine (T-Mobile LTE) still resulted in SIM settings showing "no network found" for active network.

Tried to get more detailed information via Field Test (enabled ##3282# as per here, for instance. Sadly, this appears to not be a functional option for my phone. Field test launches and gives me the "do not change any values..." warning, but after I click accept, it shows nothing other than the "field test" text at the top of the screen.)

I have only guesses as to why my phone went back to crap functionality after things went swimmingly for a day: background restore of earlier backup? Silent update of one or more Windows components a day after 1593 update was completed? Attempted silent update failed? When bad behavior set in yesterday, it was a few hours after I connected to WiFi - possibly relevant. Running Pasquale Pignataro's Device Diagnostics Hub always shows "Backup Assistant Plus" as one of the processes currently running. I'm guessing this is the non-functional Verizon thing I can't delete nor configure nor run from the control panel; but since it's a Verizon thing, it could possibly be messing with my off-network bliss? Or there is a Microsoft app/agent with the same name and it had a hand in my day-after-fix problems?

Wish my question could be more focused, but that's all I've got. Hoping some of you might have an idea of where I can start digging.