High-Speed Internet Gateway will not lock to 5G speeds (5G21)

I know this threat is getting long and aging, but I hoping for a solution. So this weekend, I gave it another try. I found 5G (B66) and found a great location, with 3 bars but with speedsno better than 700 Band. So tried one more things this morning, I did a hard rest in a location which I know has 5G was working to my surprise, I got new network connection - B2 (750) and a secondary (which I had not been getting) n41 (515790?). The speed is amazing 275 Mbps down, and 10 Mbps up. It has locked on it for about 30 minutes or so then you guessed it is drops to 700 Mhz band again. So I reset and again amazing and now waiting to see how long it last. I wish I could just manually lock in these channels. Anyone had any luck with T-Mobile customer service to resolve this issue?