Can you use Time Capsule just for wireless backup and not as a router?

Yes, but in order to do this, the Time Capsule will need to be permanently connected to your "main" router using a wired Ethernet cable connection.

So I should connect my TC directly to my router via ethernet, and then turn off the wireless capabilities of the TC? Once that is done, will my Mac devices (i have several) will find the TC over the new wireless network and backup normally?

Apple has made recent changes to their operating system that basically require that users back up to the Time Capsule over the Time Capsule wireless or via a direct Ethernet connection from the Mac to one of the LAN <--> ports on the Time Capsule.

Turning off the wireless on the Time Capsule and backing up over a 3rd party wireless router might work, but this type of setup will not be officially supported by Apple. So, if there are problems with backups, Apple will not offer much in the way of assistance.

I have also put a lot of effort into getting Time Capsule to work over wifi without an Apple router. There is sort of a workaround. I set up the Time Capsule to run it's own wifi network but don't have it connected with an Ethernet cable. It doesn’t have Internet connection. My Netgear router connects my computers to the network through Ethernet and supplies wifi for other units like iPhones, printers and a guest network. My computers also have wifi turned on but are set to connect to the separate Time Capsule wifi network. Time Machine can backup to them and I can have my Time Capsule in a secure area next door. To get this to work in Settings/network the Ethernet network has to be set as the first in the list, otherwise my computers loose access through Ethernet.