HTC Desire 820 dual sim: Updated phone and lost Mobile Data; Doesn't Connect

[GMarquis]

Since I updated my HTC Desire 820 dual sim with Android Security Patch level 2016-04-01, my "Mobile Data" no longer works. I must rely on Wifi. Phone purchased unlocked/open in Hong Kong. I upgraded the phone as instructed and cannot use Mobile Data. How do I reverse the update? Is there a patch to fix it?
AT&T has been my Internet/Data provider since purchase 2015-June.

 

[B. Diddy]

Welcome to Android Central! Have you checked your APN settings to make sure they're still correct for your carrier?