Apple - Is it possible to use an Apple Time Capsule just as a network attached backup drive and not a router?

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.
• Go to the tab labeled Wireless (not Network as zhovner suggests, as putting it in bridge mode only stops it from being a router but does not stop it from being a network access point that your devices will connect to)
• From the Network Mode pop-up menu, choose Off.
• Click Update, and the base station will restart and load the new settings.

The result will be a Time Capsule you can still wirelessly backup your Macs to, but does not function as a router or access points, so none of your devices will connect to it.

This is the setup that I use; I have 3 eero routers and thus needed the Time Capsule to not be an access point, and these steps stopped it from being that but let me keep backing up to it as usual.

From this wireless tab, just choose off in the Network mode pop-up and hit update.

Source is a Macworld article on the topic.

Yes, you can simply disable routing on TimeCapsule and set it as DHCP client.

You should actually do BOTH of the things suggested by the other answers.

1. Go into Wireless tab and set the mode to Off so that it doesn't create a WiFi network anymore.

2. Go into Network tab and set it to Off (Bridge Mode) so that it doesn't run a DHCP server/NAT anymore. Otherwise it will try to create its own subnet for any devices connected via its ports. Turning this off is very important, to avoid messing up your network with multiple layers of NAT etc (so don't listen to klanomath's comment on a previous answer).

3. Connect a network cable from your main router to the Time Capsule's WAN port (not the LAN ports). It's now a NAS (network attached storage).

Now you have achieved the following:

1. Your Time Capsule acts as a DHCP client and receives an IP address from your main router. You can reach it at that IP and connect to its network shared storage. I am connected to mine right now.

2. Any devices you connect to the Time Capsule's LAN ports will be routed "straight through" like a basic switch, WITHOUT any services/traffic modifications by the Time Capsule. The actual DHCP assignments and NAT etc for those devices will be handled by your main router (not the Time Capsule). So the Apple TC acts as a simple switch that passes through the ethernet traffic to the correct device.

So that's it. Just turn off the WiFi and the DHCP/NAT stuff and you're done. Your Time Capsule is now a basic router and a DHCP client which shares its own data folder. Done!