I was having seemingly random problems with my phone spontaneously rebooting and even getting into boot loops.
Then it seemed to go away, but I didn't realize why, at first.
Other forums mentioned that the phone seems to have problems with T-Mobile SIMs if the second SIM slot is left empty.
When I was having my trouble it happened to be when I was using a T-Mobile SIM. I had subsequently switched to a FreedomPop LTE SIM and never had another problem. It never occurred to me that the T-Mobile SIM could have been the problem.
A friend of mine recently unboxed a Pop 4 Plus that was purchased last Black Friday and had the same problem with her T-Mobile SIM. I gave her a spare FreedomPop Global SIM I had laying around and she installed it in slot 2-- and she hasn't had any problems, since. T-Mobile works great. The Global SIM works great. And no random reboots.
The phone seems to be fine with one SIM as long as it's not T-Mobile.
If you use a T-Mobile SIM then you need to put another SIM in the second slot.
It sounds crazy and I initially ignored what I'd read, but it does seem to be the case.