How do you lock a band without root?

Settings> About Phone> Tap 5 times on internal memory> Now you are in Testing menu> Select the Phone information1 for SIM 1 or Phone information2 for SIM 2> Scroll down to find Set preferred network type and select it> From the list select the one you want and go back to home.

It has 4G+ with B3 + B20 CA .