What speed does the RB5009 set the serial connection of the adapter it is connected to? 115200? What about flow control?
The
MikroTik help isn't real clear about using two serial adapters back to back.
So if you got two of the USB to RS232 cisco Rj45 cables, and a "RJ45 8P8C modular coupler with rollover wiring (1-8, 2-7, 3-6, 4-5, 5-4, 6-3, 7-2, 8-1) then it should work. Those type of coupler used to be easy to get as they were used with multi line analogue phone systems in the 1980's. Not as easy to get now.
This appears to be one.
Alternatively, two USB to 3.3V TTL wired back to back with TX-RX, RX-TX, GND-GND (and if available, CTS-RTS, RTS-CTS)
How can I tell if it will support a CH340 USB to serial adapter?