Serial console is your best chance. I found this:
The serial port is set by default to 115200bit/s, 8 data bits, 1 stop bit, no parity. Note that the device does not fully implement the hardware (RTS/CTS) flow control, so it is suggested to try to disable hardware flow control in the terminal emulation program in case the serial console does not work as expected, and if it does not help, make a new cable using the pinout given in the User's manual.
But unless you're doing it for fun, with RB133c you're wasting your time. Even if you manage to install newest available v6, it will still be terribly outdated. And the device doesn't have enough resources anyway. If it was at least RB133, that could perhaps be used for something (I know about one that still works today, because owner is too cheap to get anything better, it runs 5.26 and is sort of usable). But RB133 has 32MB RAM, while RB133c has only 16MB, and it's huge difference.