can I go back to using the wifi from my dsl modem/router? TC too slow

I had much faster wi-fi with much more area coverage from the DSL modem/router from my ISP than I now get with my TC 3tb. I have tried the 5 ghz, I have tried locking it onto a single channel, but nothing works very well. Is there a way I can go back to the modem/router wi-fi and still have the TC for wireless backup over the LAN? I hope I won't lose the backups already on my TC and have to reinitialize it's HD.

Any help on how to configure much appreciated.