I don't know if this is the solution, but it works for me. I have a Comcast AC/N gateway, I changed from stateful to stateless on IPv6 settings. I don't know what that means, but my wifi is finally working.
This is incorrect. The wifi strength is 100% not connected to the "signal" going to your modem. Even if you modem is getting a low decibel rating from your service provider, the router will be unaffected. Also If your router and modem are one in the same it will still be unaffected.