LG Flex 2 won't stay on 4t Lte keeps going back to edge network

I have an unlocked AT&T LG Flex 2 that in had originally purchased from AT&T. I decided to cancel my service with them bc my bill was always $$400 ⬆. I unlocked and rooted my phone and switch over to T-Mobile with unlimited data. I was able to enable mobile hotspot after a bit of research. But now I have an issue with my 4G LTE switching back to the edge network if the phone is not being used, or if a call comes in and I answer it switches back to the edge network. I then have to go to my settings and search network operators and it sometimes takes its sweet time to pull up the results and at times doesn't give me the option to choose RAT: LTE. I know there's a way to force 4G network but all I've came across on the net is how to do it on other phones particularly HTC and Samsung devices. Or it's on another carrier, any help would be very much appreciated.