You need to enable uart_AO_B which is disabled by default.
For example:
Enable uart_AO_B (UART_RX_AO_B, UART_TX_AO_B)
khadas@Khadas:~$ sudo fdtput -t s /dtb.img /serial@c81004e0 status okay
Note: serial@c81004e0 is for uart_AO_B.
Check the status again:
khadas@Khadas:~$ fdtget /dtb.img /serial@c81004e0 status
okay
Ok, it’s enabled now, reboot the system and you will find the node for uart_AO_B.
khadas@Khadas:~$ sudo reboot
Check the tty nodes:
khadas@Khadas:~$ ll /dev/ttyS*
crw------- 1 root tty 238, 0 Dec 13 13:12 /dev/ttyS0
crw-rw---- 1 root dialout 238, 1 Dec 13 13:09 /dev/ttyS1
crw-rw---- 1 root dialout 238, 4 Dec 13 13:09 /dev/ttyS4
So /dev/ttyS4 is what you want for uart_AO_B.