Any time you change ISPs or networking hardware, it is always a good idea to perform a complete power recycle of your networking equipment. Check out the following AirPort User tip for details. Do this for your modem and AirPort Time Capsule (TC). Please post back your results.
If successful, your TC should be back to its "out-of-the-box" configuration which is a wireless router that will broadcast an unsecured Wi-Fi network with a network name of something like: Apple Network NNNNNN. Once you verify that this has completed successfully, you can add the two AirPort Express base stations back into the network configuration. Since you want them to be "one unique wifi" you will want to configure all of your AirPorts for a roaming type network.