Try to reset SXT and reinstall by Netinstall.
Open SXT door and look for S1400 [Reset button],
This button has three functions. Hold the button, then apply power. Depending on when you
release the button, it will do these things:
• release immediately (0-5 seconds) after starting the device to load backup bootloader
• release when user LED starts to flash to reset RouterOS (5-10 seconds)
• release after user LED stops flashing to start Etherboot (Netinstall) mode
Number 2 and 3 are the options, that you have to try.
Link how to use Netinstall,