Too lazy to type; quoting my own reply. See below, J4Jesse. I'd say you un-tick bands 5 and 3 altogether after you follow the instructions below.
"I had the same issue when I learned to set this frequency set-up manually. I believe it's your modem helping you still "get" signal from the tower due to its incapacity to receive a signal from Band 28.
What I did is, I took a very long extension cable, plugged in my modem, then placed my modem in every feasible corner where it can collect the signal.
Please note that I kept the frequencies locked to Bands 3 and 28 while scanning. By default, you will see Band 3 locked in the system. Then, after reaching RSRP of equal to or more than -100 AND SINR of over +5, keep it to where it's placed at that very moment, reboot your modem, and you should see the band change to Band 28 with better internet speed - at least this thing happened to me, may not be applicable to everyone."