Unfortunately u-boot image versions are currently not consistent across the various TO hardware revisions and thus
5 LEDs: Unsecure SSH on 192.168.1.1 (Omnia 2019 and newer)
is not available on CZ11NIC13 | CZ11NIC20.
has not been responded to by developers.
You would need to gain terminal access (ssh) via ethernet cable, in lieu of a serial cable, to the device in order to modify the bootloader environment variables, that if you want to return to TOS instead of OpenWrt.
The boot env reset command from ssh is slightly different than with the serial connection (not sure whether it is supported since it requires a binary from the uboot-envtools package):
fw_setenv bootcmd 'env default -f -a; saveenv; reset'
After reboot you could then attempt TOS medkit installation.
OpenWrt provides documentation on reset | rescue | recovery from botched firmware installations that may provide a way of connectivity other than the serial cable.