Time Machine on iMac not backing up to Time Capsule

This was going well until a couple of weeks ago, when my router failed and I lost connection to the internet, but that may only be a coincidence.

Essentially, TM no longer backs up my iMac to my TC. I know it's old technology and will not be supported with the next release of macOS, but my iMac Pro won't support anything later than 15.7.2 (24G325) anyway, so I might as well continue to use it.

I didn't realise it wasn't backing up the iMac until yesterday, because it didn't notify me. It just has the smallest ! in the tiniest icon on the toolbar, but now I do.

This is what I found I finally looked at the problem:

This statement (error message) isn't true, as far as I can tell, because nothing within the network has changed, and the iMac is hardwired into a network switch, into which the TC is also plugged in to.

As all my data is stored on a NAS, I'm not over concerned about this (I think) so I deleted the old backup files and reconfigured TM to the drive, but the same problem occurs. This time the error is:

It seems to be a network issue, but my router app shows all the devices are connected, I can see the TC in Finder and the Airport Utility is showing a green light, so I'm not sure what's going on.

Would someone be kind enough to point me in the right direction, please?

Is it actually worth backing up the iMac, if all my data is on the NAS? Just throwing that in there...

Thank you.