The TC is not made to be used with wireless internet. It is really a pain as Apple have made next to no provision for it to be used other than by ethernet.
There is a join a wireless network mode.. unfortunately the airport utility is prone to bungle this, and you have no choice on the latest AC model but to try try try again. The TC should be able to join your existing wireless network, and exist as a wireless client on the network.. with the following caveat emptor..
1. The TC will now be super slow.. all packets are passed via the existing wireless network to the TC.
2. The TC will stop having any use.. it is now a dumb client.. your fancy AC wireless is reduced to client at whatever speed the wireless router is working at. The ethernet ports are turned off.
3. You will overwhelm the network when you do backups as the bandwidth provision from the wireless internet router will be too slow. And without QoS to push background backups to where they should be.. background. All clients will be affected.. pushing bulk backups over wireless is a bad idea.
4. If the wireless internet is not in your hands.. ie the router is outside your home.. do not do it this way. They will have it setup to specifically limit your usage.
We strongly recommend an avoid at all costs.. this is a lousy poor setup.
In your profile which you have informed us is wrong.. do you actually have a Mac Pro.. if so, keep the TC in router mode and use ethernet. Tell us the details because I have written it up a few times.
eg https://discussions.apple.com/thread/4817218?tstart=30
The TC btw in this setup is the wrong product.
Or if you can get a wireless router with ethenret ports, there are plenty of wireless routers around that take 3G/4G wireless sticks directly or even the SIM.. then you can bridge the TC to it.. far superior to using a wireless hotspot type product.