Instructions to use Time Capsule with MacBook Pro (Catalina)

I'm not techy, just saying that right out of the gate. I've tried following several (old) instructions on using my AirPort Time Capsule as backup ONLY with my MacBook Pro running Catalina 10.15.7, but I'm not having any luck. In our previous home, we used the AirPort as our router, but not here. I only want to use it to wirelessly backup one Mac. (Maybe two, if I can get it to actually work.)

I connected the AirPort to the router with a WAN/LAN cable and updated the firmware. When I open AirPort Utility, it shows the device as being connected to the internet. It's also visible in Time Machine Preferences. When I try to backup, it looks like it's starting to work (preparing disc, etc) but then fails and this is the message I get:

"The backup disk could not be found. Make sure the backup disk is connected or select a different backup disk."

I've searched and searched to find the answer, but have failed there too. The most recent advice that looks like it might be helpful is a decade old and doesn't match what I see on my screen. Can anyone help me figure this out?