Unlocked S8+ (Tmo variant) work on other carriers?

[AC Question]

I have an S8+ on T-Mobile and wanting to get it unlocked through their unlock app and move it to another carrier like Verizon or ATT. I get zero reception at my new house.

Is anyone in a similar situation and confirmed it works on other carriers? If it does how does the T-Mobile firmware switch to ATT or Verizon, does it automatically switch over after factory reset or you'll have to flash it someone?