I tried all the methods posted at WPCentral. Actually, mine got worst - Now, it doesn't show any data status - LTE, 4G or 3G.

I am seriously thinking either to move back to Samsung Focus I had or replace in the store with a new one. 

I don't know if it's an issue - I am on grandfathered unlimited data plan. ATT was able to move me to equivalent Unlimited Plan which I checked on my online account - LTE Unlimited Data Plan.

I am wondering if ATT is going to resolve as well as compensate for all these trouble with the service migration. Anyone had any luck with this?