@MorrisFury 

Hi

Ethernet connection cannot be a backup for 4G mode. 

TP-Link 4G LTE Router supports 2 operation modes:

    1. 3G/4G Router Mode: The outer requires a SIM card to access the internet.

    2. Wireless Router mode:  The router connects to the internet via Ethernet Cable and the WAN port.

If you wish to connect the TP-Link 4G Wi-Fi router to an existing router, you may change the mode to Wireless Router mode. Under the Wireless Router mode, you could use a 3G/4G network as a backup solution for internet access. When 3G/4G Backup is enabled, your router will fail over to the 3G/4G network when the original network service fails.

For more detailed instructions, please refer to this FAQ: How to set up Wireless Router mode and 3G/4G Backup on TP-Link LTE Gateway

Thank you.