Just in case you're still looking for an answer...
In order to enable a uart you have to grab the relevant dts file from:
https://github.com/radxa-pkg/radxa-overlays/tree/main/arch/arm64/boot/dts/rockchip/overlays
They're all in rk3568-* for some reason.
For example if you wanted to enable uart4 on pins 16/18 you'd grab
https://github.com/radxa-pkg/radxa-overlays/blob/main/arch/arm64/boot/dts/rockchip/overlays/rk3568-uart4-m1.dts
Then load it with armbian-add-overlay.
amrbian-add-overlay rk3568-uart4-m1.dts
and reboot.
Yannic van Moerkerk reacted to alansrobotlab in UART not working