Question about TP-Link RE700X

Yes. It works fine plugged into the mains socket. No other cable is required for normal operation or setup.

Thank you!

There are 2 modes:
- WLAN extender: An existing WLAN is picked up and amplified again
- WLAN access point: A LAN cable is connected to the device and a WLAN is created.

I suspect that the cable is a network cable that connects the TP-Link to another device that needs to be connected to the network according to your individual configuration.
The best thing to do is to call the tp-Link hotline: 0211 54474970