Yes, this is possible and actually quite simple. To prevent the Time Capsule from acting as a WiFi access point or router, and use it only as a backup disk, just open the AirPort Utility app on a Mac (Applications -> Utilities -> AirPort Utility) and do the following: Select your Time Capsule and hit Edit.
The AirPort Time Capsule (originally named Time Capsule) was a wireless router sold by Apple Inc., featuring network-attached storage (NAS) and a residential gateway router, and is one of Apple's AirPort products. They are, essentially, versions of the AirPort Extreme with an internal hard drive.Apple describes it as a "Backup Appliance", designed to work in tandem with the Time Machine backup macOS: Archiving a Time Capsule - The Mac Observer Interesting timing – last night I was using (or at least trying to use) this feature on my Time Capsule to create my “off-site” backup copy – which I do every 6 months or so. Amazon.com: Customer reviews: Apple AirPort Time Capsule
Is it possible to use an Apple Time Capsule just as a
Automatic wireless backup. AirPort Time Capsule includes a 2TB or 3TB hard drive that works with Time Machine in OS X to create the perfect no-hassle backup solution.1 There are no cables, and you can back up all the Mac computers in your home to a central location — automatically, wirelessly, and in the background. Ultrafast 802.11ac Wi-Fi. Discontinued AirPort Extreme and 2TB Time Capsule Finally Nov 16, 2018
Apple AirPort Time Capsule review: Fast Wi-Fi and easy
Airport Time Capsule And Time Machine Backups– An Inside That’s all about AirPort Time Capsule and Time Machine backups. The concept is almost the same with slight differences and Time Capsule offers you a bit of extra security by keep all your Macs connected to the network backed up. Nevertheless, you must keep an additional backup source as well.