If you would like to use a UART that is not the main debug UART, the overlay GB-UART2-FLX4-AD2-AD3.dtbo will enable a second UART on pin AD2 and AD3. You can enable the second UART by editing the uEnv.txt
Make sure to uncomment the line
enable_uboot_overlays=1 this allows overlays to load on boot:
# enable u-boot overlays enable_uboot_overlays=1
Then you can add the line GB-UART2-FLX4-AD2-AD3.dtbo to one of the pre-existing slots.