The OpenWrt forum has a thread from a user asking the same question, which confirms that OpenWrt does not support the THG3000.
If your router experiences dropped connections, slow speeds, or interface lag after an update, use these steps to resolve the issue. 1. Perform a Hard Reset thg3000 router firmware
Connect a laptop or phone to the THG3000 via Wi-Fi or Ethernet. The OpenWrt forum has a thread from a
A failed flash can brick the router. Use only wired connection, no power interruptions. Perform a Hard Reset Connect a laptop or
Never unplug the power cable or press the reset button while the router is flashing during an overnight update. Interrupting this process can corrupt the flash memory, completely bricking the device. Unleashing Advanced Features via Expert Mode
If your router has not updated automatically, you can trigger a check through the administrative settings: Advanced Settings : Log in and select Advanced Settings Update Check : Go to the tab and click
Push a paperclip into the hole and hold it down for 15 seconds.