A lot depends on the setup.
Wireless is less than great.. and even very much worse is "Join a wireless network"
I positively discourage people from using it. So correct setup is essential.
The TC MUST be plugged into the main modem by ethernet.. it can be in either router or bridge mode depending on other network elements.
There is a good chance nothing much is actually wrong.. Plug the TC into a computer by ethernet and do a check of the TM sparsebundle. If you do not mean TM.. files can often be recovered by removing the hard drive.. but Apple is right for TM it is not work the problem.. wipe and start over. TM is designed as a current snapshot.. any historical recall of data is not to be relied on.
There are other issues that can be fixed or helped by proper network setup. But Apple's latest firmware was not a great leap foward.. more like 2 steps forward and 3 or 5 backwards.