Hard to say what could have happened here, but the way timeout works in RutOS, is that after ~300 seconds of inactivity you will be logged out and brought back to the login screen. The device will still be reachable and you can re-login, so it seems like another issue is at play here.
Since a factory reset was performed on the device, I assume not much configuration was done on the device. In this case, I’d suggest trying to perform a bootloader recovery procedure described here: https://wiki.teltonika-networks.com/view/RUT9xx_Firmware_via_The_Bootloader_Menu
Keep in mind, that if your device shows an error after trying to upload the latest available firmware, you may need to install the latest legacy firmware (RUT9XX_R_00.06.09.5_WEBUI) and then update to the latest version using the WebUI of the legacy firmware.
If that does not help, could you clarify if the issue is only present when using an Ethernet cable, or does the same happen when using WiFi?