For changing bands, you can use UCI commands.
First of all, go to your router and check if with CLI you can find the uci syntax for the line where bands are saved.
uci show | grep b7 (or use anything that you've set recently including b20, b38)
This should output you a lot of lines and one of them should look like this:
simcard.@sim.lte='lte_b1' 'lte_b8' 'lte_b41'
But it could be different in your situation.
Then you just use uci commands to set new values, so in your case we want to do it with SMS:
<yourAdminPassword> uci set simcard.@sim.lte='lte_b3' 'lte_b7' 'lte_b20'
then send another sms:<adminPassword> uci commit
and after 10 seconds send this: <adminPassword> luci-reload
This should change your bands.