I was reviewing the RBM33G user manual.
And besides the basic GPIO, it has CTS/RTS and RX/TX UART, which leads me to believe that it supports RS485, and therefore would support MODBUS.



Is my reasoning correct?

If yes, this RB would be PERFECT for an Out-Of-Band Router.

It's probably RS-232 3.3V TTL and you should be able to run MODBUS over it yes. RouterOS has built in "Terminal server" capabilities. But it will not automaticlly be MODBUS-TCP (port 502)

You can use it as transparent serial to tcp bridge. If you add RS485 convertor it may do MODBUS or any other protocol.
But don't expect any MODBUS functions from ROS like on KNOT, it will only really do the raw bytes forwarding from/to UART port, no data manipulation.