Bridging Two UARTs on Same SBC
De-compile the .dtbo file regarding UART enabling with DTS compiler. Then download the CPU manual and look for GPIO section. From board scematics, write down those involved pins and look in de-compiled .dts file what setting they provided: pull-up, pull-down, none  or if the output is open drain.
Well if TX pin is open drain and RX pin has no pull-up, then you get nothing.
my example:

find the ff190000 (uart1)

find what's behind the phandle 0x46

0x03 means GPIO3 bank, 0x0C is the pin number, 0x02 probably means special function (not a standard GPIO, rather UART pin) , 0xB3 is pullup setting :

So for the next pin: 0x03 = GPIO3, 0x0D = pin nr., 0x02, 0xb0

EDIT:
Now if you look at the RockPi 4 schematics you will notice that UART1 pins are already used for Ethernet controller, so it can be used. This is the reason why you see in the DTS file it's &quot;disabled&quot;. You should get a conflict warning in dmesg if you try to enable it.