i dont think gpio overlay are available in armbian see post: Cant enable overlay on Armbian_22.08.0-trunk_Radxa-zero_focal_current_5.10.134_xfce_desktop.img.xz
you need debian, once you have that you can use : https://wiki.radxa.com/Zero/Debian to enable the overlay you need for UART_AO_B and UART_EE_C
I’m not an expert tho