Apparently, when using a tethered network connection using your iPhone 5, there are issues logging into ancestry.com directly via your favorite browser or FTM 2012 software.
Of course, ancestry support indicated that the problem is due to AT&T's network. Go figure! The only problem with that arugment is ancestry.com is the only issue I see using LTE. No other web site has an issue.
Now, if you connect via a VPN the issues go away.
I suspect there are issues with the security software that acnestry usses to manage there web access.
But trying to tell this to a bunch of arrogant support people who apparently do not have a computer background.