Tim2060

Sounds like display driver crashing. If you reinstalled everything, it's probably hardware related. Try a different PSU if you have one and check that the pcie cables are plugged in properly. If that doesnt fix it I'd memtest it.

Put the sim in your phone and check which band of LTE is available. (I like using the app "CellMapper". No bloatware etc) Cell carriers like to use different bands in different scenarios, so saying that the carrier uses one band specifically isn't practical. Unfortunately band support is a hardware feature of the modem, so you're not going to be able to change that.

Is the thermal paste actually spread when you take the cooler off? Sounds like there is a problem with heat conductivity somewhere. (Cooler cold) If not, sounds like the S-TIM has somehow gone bad. Maybe also post a screenshot of HWInfo or the PC (both?!).