I've got this same problem on my K790a. I had it unlocked for about a month, went to Brussels, and it worked perfectly. Then, when I came back to the states it worked for about 3 weeks, but suddenly it said "No Network Coverage." I've tried reflashing it with several different firmware revisions & locations, and I even bought a new replacement antenna module, and that didn't fix it. Anyone have any ideas? What was this about an IMEI being blocked? How can I see if that's a problem for me?

Thanks in advance!