You can buy routers for 3G/4G connections.. it takes but a few minutes on google to find them.
Search for 3G router.
An even better place to search is ebay. You don't need to buy there but it will very quickly give you a full look at the range of products available.
What is important for you is having ethernet port. So make that part of the search.
The one that pops up most in my area is
So it does not take a SIM directly. What it does is use USB 3G modem.. as long as you make sure you buy the right one ie on the compatibility list it will work fine.
Not many 3G routers use inbuilt modems.. they do exist and I have played around with a few but they are often a lot more expensive.
Another solution is buying a cheap router which has a USB port and run it on 3rd party firmware.. many routers are now available that have USB 3G fallback.
See ROOTER project for more ideas.
http://ofmodemsandmen.com/
The other way around the issue.. since the TC must use ethernet connection to WAN port to be useful is to buy a hotspot.. you will find loads of these around.. and of course even your phone could have a hotspot mode.. ie internet connection via 3G to wifi for local LAN.
You then need a wireless bridge.. the obvious one is airport express.. but any wireless bridge would do the same thing.
That gives you an ethernet connection to the TC which you will run in bridge mode to the router in the hotspot.. or double NAT to your local network.. the problem here is that double NAT. your main wireless modem on the roof might give you a far better connection in terms of routing even if the speed is off.
