AT&T Uverse residential gateways (or routers, if you wish) generally have terrible built-in wifi. Combined with the fact that mobile devices typically do not have the best wireless antennas and you’re living in an apartment, wifi reception or signal degradation due to neighbors around you could be a primary cause for buffering issues. You need to try changing the wireless channel in the settings for your residential gateway to one with less interference.
If that doesn’t work, check to make sure that you can watch Twitch normally on a computer connected directly to your residential gateway. By directly, I mean with an ethernet cable. Ethernet is the only true way to test that your internet is functioning properly in this case. If on ethernet you continue having issues, you would need to reach out to AT&T support to help diagnose your internet issues.
Further, these forums are meant for developers seeking help integrating Twitch in their applications, not for general support. In the future, you can reach out to Twitch support at http://help.twitch.tv (“Send us an email” at the bottom of the page).