I can't backup in Time Capsule without WiFi

Same answer. It is not a wired client on that network. Find out how to add a wired client to the TP-link mesh system. Consult its documentation. I have no idea whether it is even possible to add a wired client to that system.

When I try to turn off the WiFi on the Time Capsule as I don't need another network, I am not able to make backups.

Then it is not properly connected to your network.

You probably need to reconfigure the TC by starting with a "hard reset": How to reset your AirPort base station - Apple Support. Connect it to your network with an Ethernet cable, disable the TC's wireless radio, and configure it as a (wired) client.

After it resets, you will need to "reconnect" to the TC by removing it in Time Machine's settings, followed by adding it again.

None of the above affects your existing backups.

Hi,

You can connect the Time Capsule via ethernet to the rest of the network via its WAN port, and it will be available to any computer that connects to the network, whether that Mac is using Wi-Fi or not. In AirPort Utility, you can disable a base station offering Wi-Fi service.

If you turn off the wifi on the time capsule, you will also turn off the ability to receive, so if you're backupping wirelessly it needs that feature