We need more information here, please.
We know that you have a Time Capsule, but your post did not tell us what other router you have. We need the make and model number of that product.
Are you planning to "extend" by connecting the Time Capsule to your other router using a permanent wired Ethernet connection.....or......are you planning to extend the network wirelessly?
Which device is your "main" router? The Time Capsule or the other router?
Apple's "extend a wireless network" feature appears to be proprietary to Apple.....and as far as we know....will only work when the Time Capsule connects wirelessly to another Apple wireless router.
So, it would be extremely unlikely that that the Time Capsule could extend the signal from your cable/modem wirelessly.
Instead, to allow the Time Capsule to extend the wireless signal coverage, you will need to establish a permanent wired Ethernet cable connection from your modem/router to the Time Capsule.
Then, configure the Time Capsule to "create" a wireless network using the same wireless network name, same security settings and same password as your modem/router network.
As a final step insure that the Time Capsule is configured to operate in Bridge Mode to work correctly on the network.