rextended is right, basically if a Serial adapter support Linux without add’l drivers, it should work. If it requires a driver or installer, the serial adapter can’t work on RouterOS.
The only WAG in V7 is to set “serial” under the Interface>LTE>Modem settings, since I guess it’s possible LTE might be trying to use it BUT I doubt it.
I had tried using Prolific usb to serial chips in the past and they were nothing but trouble. Fake chips everywhere, old chips not supported by new drivers, newer chips causing windows driver to BSOD on removal, chips dropping data at higher baudrates, … etc etc.
Prolific = Avoid at all costs!!
If you want proper brand name adapter, buy FTDI.
If you want something cheap, even CH340 is better than Prolific.