Thank you for utilizing the support forums! I understand how not being able to connect on your iPhone can be frustrating. The data speeds that you stated you have are well over what you need to connect & stream so the speeds shouldn't be an issue. If you are using the Slingplayer application (which is the only way to connect to your Slingbox on an iPhone) then you could try uninstalling the application & reinstalling it again.
Below is a link that will show you how to reinstall the application once you uninstall it.
Also what you could do is try to connect over a wifi connection. If you can make that connection on a wifi network then you will know for sure that something with your network is stopping you from connecting.
Feel free to reply with any questions or comments!
The Sling Moderation Team
here is the final deal........AT&T BLOCKED SLINGPLAYER FROM PLAYING OVER 3G, 4G, LTE, GOOGLE IT, AT&T ISSUED A STATEMENT SAYING SO. pretty much slingbox is completely POINTLESS now. why would I want to watch TV in my own home on my phone when I have a 50inch in front of me......its not worth the $$ to pay to "just" be able to watch it in a wifi spot somewhere else...sucks because slingbox was very cool a few years ago when this was not a problem..