Buy the unlocked iPhone. I use the Straight Talk $45. Plan - unlimited talk and text and 10 GB HI speed data. The sim package comes with 3 SIM cards so you can choose which card. Have used for 5 yrs. I’m done with contracts.
My 6s I have now (waiting for delivery of my 8*) shows LTE when not on WiFi. T-mobile is one of the SIM cards that comes in the Straight Talk kit. Works fine for me.