I had an iPad connection problem as well - possibly the same one. My iPad found the router just fine, but always said "Unable to join the network "EVO xxxxxx" (where the x's are my router's numbers). My only option was to select Dismiss.
I called Sprint and the very nice tech did all he could, but couldn't resolve the issue either. But while talking through the process I saw that something was in my Notifications Panel on the phone. When I pulled up the message it said "Connection requests received" -- indicating that the iPad did try to jump on. I tapped that message and it took me to a screen that showed I had turned on "Allowed users only" in the Hotspot settings. Just for grins I tapped the checked box. That then gave me the option to "cancel" that feature. Lo and behold it all worked after that.
I don't quite get the "Allowed users only" feature in retrospect because there was nowhere on my phone to say who was allowed. The Sprint tech said his printout showed further options to define that. My Hotspot setting screen did not. So "whatever" because it now works for me. I do have security option WPA2 (AES) set with a password so I'm guessing I'm ok. I'm not technical enough to know if I'm at risk for others trying to jump on my connection, but I'm just happy it works.
Hope that helps...