AirPort Time Capsule configuration
I have WiFi but no access to the main router. Can I configure my AirPort Time Capsule as a simple backup?
iMac 21.5″, macOS 10.15
The Time Capsule needs to connect to the "main" router using a permanent wired Ethernet cable connection if you want the Time Capsule to be on the same network as the router.
In other words, you cannot configure the Time Capsule to connect to your existing Wi-Fi using only a wireless connection.
If you cannot connect the Time Capsule to the main router using a wired Ethernet cable connection, then your only other option would be to set up the Time Capsule to create a wireless network and back up your Macs over that Wi-Fi network.
The downsides......you will have to manually switch your Macs from your existing Wi-Fi network to the Time Capsule's Wi-Fi network to back up, then once the backup has completed, you will have to manually switch your Macs back over to the normal Wi-Fi network. The reason......the Time Capsule Wi-Fi will not have Internet capability.