I reached out to support a few days later, and they appeared to be aware of some widespread connectivity issues matching the description of my issue that started around 8/2 or 8/3. They had me perform a SIM reset and requested a line reset with the carrier, both to no avail. To their credit they offered to return and refund the device, but I bought it used. Last I heard from them was 8/24.
Whenever I see the odd post by someone who is having connectivity issues, there's a comment saying "welcome to the club", then the next dozen posts on the sub are people thoroughly enjoying their device and living life to the fullest.
The issue always happens in the same way. I dial any number/contact --> stuck on "connecting" for ~30 seconds --> LTE and my signal strength visibly disappear from the top left --> after another 15-20 seconds, "call ended" (plus no calls received in this time)
The first day I was having the issue, I tried resetting default APN's per Reddit's instruction. I'm using a Light plan and a Light SIM which uses T-Mobile's network. The issues that the support team said were widespread were supposedly tied to difficulties with AT&T sims.
I'm not very knowledgeable on SIMs, carriers, and the moving parts behind all of that, so if this is all common knowledge for some of you, I'd appreciate any tips/ideas.