?

Just reread your post, I think I misunderstood it the first time around.

The AUX port on the Cisco is just an async serial port, right? So what you are suggesting, if I read correctly, would be:

a) plug a serial device (computer or whatever) on to the AUX port of the Cisco router
b) configure x25 routing on the Cisco so that incoming X.25 connections via XOT go to the AUX port

Would this be correct?

If so, that would allow me to join TELEBAHN quite easily :) I guess the computer to connect could be an ESP32 running a CP/M emulator, there must be software from that era available.