I think it's ATT. ATT have this problem of not thoroughly testing their brand new device to be on sale to find out if one has a problem and that's also the reason why only major feature updates from developers are being pushed by ATT but not bug fixes.
Relax guys its not Nokia's fault. Its the fault of at&t.the reason for data connection loss is no apn(access point name) update is made by at&t for lumia 900. Its not the manufacturer's fault.
Look Nokia AT The Most Popular Phone till now , It Is Nokia N9 Running Meego , Why to follow WP OS -.-
Meego is amazing OS.
My goodness the amount ive read over the years about AT&T network is could be them at fault..Another thing is LTE is new tech as well..
many of the people blame AT&T's network. well i think the same.
btw, i wanna have lumia 900 despite this issue.
ICS 4.0.3 is also having many bugs that some, but definetly NOT all, claim to fix by doing a factory reset! Microsoft always release their software before it have been properly tested, is still only a beta or alfa, because that is how they have always done it on PC where the user have to wait and install bugfixes... as updates! Unfortunatly now Google have adopted the same and release their software before it is really ready to be released, and many have problems with wifi and overheating and app-incompatibility... on the first release of ICS, 4.0.3!
Users experiencing problems can try suggestions found here
Not gud situation for nokia at the time when they want win d hertz of ppl
I think it's an AT&T issue not Nokia's , i had similar issue with HTC Titan, had to hard reset before i get connectivity.
Try either of 2 choices
-Just go to market place and download network setup by nokia. that should fix it
-Remove the SIM card > Perform a Master Reset (Settings > About) > Boot up without the SIM card > Turn off the Lumia 900 > Verify that the SIM card is LTE-enabled > Insert SIM and boot up.
Be careful Nokia, or u will b in history, its ur last chance to stand on your feet in this monster crucial market.