Mikrotik - enable rb750r2 uart

I was given a few mikrotik units to be ported to ath79. built a small setup that allows me to boot initramfs images using the ar71xx target. now i would like to start porting them to ath79 but am failing to get uart up on the rb750r2. does anyone know which gpios are used for uart on these pins ? looks like pinmux needs to be setup before i can continue. also is it possible to make the "try-ethernet-once" thing persistent ? its a bit of a PITA to fire up that webfig ui every test cycle.

Its not pinmux, its the no-uart that is passed by the bootloader.
I have only seen it on RB750r2 series while running ar71xx