Trouble finding on-board serial port via UART: "-bash: echo: write error: Input/output error"

I have an old tv box that I wiped recently with Linux. I would like to connect a device to its onboard UART pins, to which I've soldered wires that connect to a UART adapter on my PC. I opened up a PuTTY serial monitor that reads incoming data while simultaneously trying to send data from the tv box. But whenever I try to interact with any of the ttyS* ports, I just get -bash: echo: write error: Input/output error. I've tried using echo, screen and minicom, but all of them seem to spit out the same error. While messing around with setserial, I get these kinds of error messages:

root@arm-64:~# setserial /dev/ttyS1 uart 16550
Cannot set serial info: Invalid argument

I'm totally unsure what to do, it seems almost as if it's none of these four ports, but I don't see another option for UART. I'm looking to at least verify the port name that my device is connected to so I can use it down the line. TV Box model: Tx mini 3 s905w processor