Retrieving data from an outdated Airport

I had a 2015 imac and when the operating system upgraded I could no longer access my Quickbooks for Mac 2015. I bought a new iMac and the QB data couldn't migrate over to the new QB online because I couldn't access the data with the latest operating system my new computer uses. I didn't despair because I have a backup file on my also very old Airport Time Capsule (2015).

Unfortunately, my new computer doesn't recognize the Airport and I don't know how to retrieve those files. Applecare representative said the Airport is no longer supported. Anyone know how I can access the files on the Airport and retrieve my last QB backup? I'm getting panicky thinking I've lost years of QB data.

I welcome any suggestions!!