My case worked, because there was a Woobm on site. Somebody plugged it in, and I had console access via other MT router.
http://forum.mikrotik.com/t/woobm-usb-how-much-can-it-do/112274/1
USB/RS232 serial cable should work as well, if loccked out from all ethernet ports (eg bridge VLAN filter enabled)
MT Woobm is discontinued. Will have to built your own (costs €7/$7 for ESP32 development board these days)
https://atadiat.com/en/e-esp8266-esp32-wifi-serial-bridge-converter/
The first USB-UART converter is on the ESP32/ESP8266 development board, just as is the wifi hardware, power comes via this USB.