RUTX12 Bootloader menu and re-install the firmware.

take off power cable from the device

set a Static IP on your PC, e.g. 192.168.1.5 and use 192.168.1.1 as default gateway

Plug in one Ethernet cable from your computer into any LAN port

Press and hold the reset button

While holding the reset button, plug the power cable into the router. Wait around 4s until all LAN LEDs start blinking simultaneously, then release the RESET button;

to enter the bootloader's WebUI, open your browser (Chrome, Firefox, Opera) in Private mode and enter http://192.168.1.1/index.html;

once in bootloader's WebUI you can upgrade the firmware using this version https://wiki.teltonika-networks.com/wikibase/images/2/2b/RUTX_R_00.07.02.7_WEBUI.bin