Upgraded to 4.3.5 and now have no cellular. Full bars, AT&T, 3G are showing. Wi-fi is OK but not cellular. Tried different suggestions from google search (Turn off/on, toggle airplane mode off/on, toggle roaming off/on, reset network settings, reset all settings)

Called AT&T support and ran thru basically all the above again. They "re-registered" my data plan for the iPad, no success. AT&T even had me Erase all content and settings (basically back to out-of-the-box iPad) no change.

iPhone upgraded to 4.3.5 and no problem with 3G, just iPad

I think it is caused by 4.3.5 on iPad only. Any suggestions?

It was the Sim card.

Took iPad to Apple store, they ran diagnostics and restored to latest iOS (even though it was up to date). Took sim card from iPad and tried it on their phone - same "could not activate cellular ..." message.

Swapped in new sim card at AT&T store - works great.

If you get this message and have another Apple product with a sim card, try switching in a sim card from your other device its a quick easy test.

You would think that AT&T support would have known this!

Thanks for letting us know. I guess it was the coincidence of the SIM card failing at exactly the same time as your updated iOS that threw them.

Tim

before reading the solution i was going to ask you if you dropped the ipad possibly causing the sim card to loosen. That happened to me.