You will need to perform a Hard Reset on the Time Capsule, as it will need to be reset back to factory default settings before it can be configured with the new settings that it needs to work with your existing modem/router.
The Hard Reset will not affect any of the data stored on the Time Capsule hard drive. However......if you have "upgraded" your Macs to Catalina, unfortunately a number of users have reported that their older backups are no longer visible.....since Apple has made a number of undocumented changes with Time Machine in the Catalina operating system.
Bottom line......we can't predict whether or not things will work or not with your old backups, but you should be able to start new backups from this point moving forward.
To perform the Hard Reset.......power up the Time Capsule (TC) for a few minutes......if it is not already on.
Hold in the reset button on the back of the TC for 7-8 seconds and release
Allow a full minute for the TC to restart to a slow blinking amber light status
On your Mac.....
Click on the WiFi menu item at the top of the Mac's screen
Look for a listing of New AirPort Base Station
Click directly on AirPort Time Capsule....or....it might read simply Time Capsule depending on the model of TC that you have
When you click on Time Capsule, that will start up the AirPort Utility setup "wizard" automatically. The wizard will take a few seconds to analyze the network and then display a screen that will look similar to the example below, except that you will see a Time Capsule pictured.
Wireless Network Name.......Type in the exact same wireless network name that your modem/router is now providing
Base Station Name.......Type in a device name that you want to call the Time Capsule. Example.......Time Capsule
Password......Type in the exact same password that the modem/router WiFi network is using
Verify......Type in the same password again to confirm
Click Next
The wizard will set up everything for you automatically. When you see the message of Setup Complete, click Done
And.....you are done with the setup
The Time Capsule is now providing extended WiFi coverage for your modem/router WiFi network.
If you are using Time Machine to back up the Mac, try to run a backup.