Q: How can I use my time capsule as a backup but not use the wifi

You can do this, but be aware that Apple does not support Time Machine backups in this configuration.

If you wish to press ahead, you basically need to do the following:

• Connect the Time Capsule (TC) to the network with an Ethernet cable. This can be another router or to an Ethernet switch. Regardless of which you use, the TC must be on the same network as the devices that will need to perform the backups.
• Reconfigure the TC as a bridge.
• Disable the TC's wireless radios.
• (Optional, but recommended) Assign the TC a static IP address or create a DHCP reservation for it so that it has a consistent address for Time Machine clients to find it.

How can I use my time capsule for backup without connecting to wifi?

Sorry, I am not clear on what your question is asking.

I assume that the Time Capsule will be connected to your modem or modem/router using a permanent, wired Ethernet cable connection, correct?

Do you want to back up your Mac without using the WiFi on the Time Capsule? (And instead use the WiFi on another modem/router that you might have to back up to the Time Capsule)

or

Do you want to back up your Mac without using the WiFi on your Mac?

or

Do you want to do something else?  If yes, please provide more details.