The band appropriate to the LoRa concentrator card shipped in the gateway is stored in one of the initial MTD partitions along with things you don’t want to loose, like the MAC address.
It’s then used in one of the lua scripts (lorawan.lua) that implements the luci web configuration GUI.
Please consider the following only if you are deeply familiar with Linux (and ideally OpenWRT) and willing to risk the continued operability of your unit - this is not typical usage!
If you want to change it, the best solution might be to modify the logic of the lua script to ignore the flash setting and give you the band you want (or give you all the choices); that way if you mess up, reflashing the firmware would revert to stock behavior. Wheras if you changed the MTD partition a mistake might have greater consequences (and if you changed the wrong partition, you could damage U-Boot effectively bricking the system in a way that would require JTAG or desoldering the flash to fix)
I forget the name of the tool used to read the configuration partition, but the lua script makes it clear; once you’ve found that do a little searching for the filesystem for anything else that might use that tool and need to be changed as well.
Beware that accidentally telling a concentrator card to operate on a band it isn’t built for may not be healthy for it.