can i use only as a back up drive and not a base station or router

Start the same check sequence as above........

Open AirPort Utility on your Mac (Finder > Applications > Utilities > AirPort Utility)

Click on the Time Capsule to select it

Click Edit in the smaller window that appears

But now click the Wireless tab at the top of the AirPort Utility screen

Check the setting for Network Mode

If the current setting for Network Mode is Create a wireless network.......the Time Capsule is providing a wireless signal.

If you want to turn off the wireless function on the Time Capsule........change the setting for Network Mode from Create a wireless network to OFF, click Update at the bottom of the window and give the Time Capsule a minute to restart.

Note.......Do NOT change the setting to OFF......until.....you have the Time Capsule connected to the main Eero station. So, an Ethernet cable will connect from your modem to the WAN Ethernet port on the back of the main Eero station and the Time Capsule will connect to the other Ethernet port on the back of the main Eero station.

Check to see if your Mac can now backup over the Eero wireless signal.

Yes, assuming that you are asking about a Time Capsule. (Your post did not indicate what product you were asking about).

Your next question might be "how' do you do this? If our guess is correct, we need some basic information.

1. What is the make and model number of your current modem/router?
2. Will you be using a Mac or an iPhone / iPad to reconfigure the Time Capsule?
3. Are you OK with a permanent wired Ethernet connection between your Time Capsule and modem/router?

Chances are, if you are currently using a FIOS gateway router with your Time Capsule, the Time Capsule is not performing as your network router.......despite what you might think.

What you may be meaning to tell us is that the Time Capsule provides your wireless network signal, which it can do without being the network router. To see if that might be the case, you can perform the following check:

Open AirPort Utility on your Mac (Finder > Applications > Utilities > AirPort Utility)

Click on the Time Capsule to select it

Click Edit in the smaller window that appears

Click the Network tab the top of the next window

Check to see what the current setting for Router Mode might be

If the current Router Mode is "Off (Bridge Mode)".......the Time Capsule is not acting as your network router. The "Off" means that the routing functions on the Time Capsule are turned "Off".....and the FIOS gateway is acting as your router.

If the current setting for Router Mode is "DHCP and NAT"......the Time Capsule is acting as your network router.

What setting for Network Mode do you see when you run the check above?

What you really might be asking is how to turn off the wireless function of the Time Capsule.....because you want to use the Eero wireless.

Might that be correct?

Nice work. Happy to help.