I'll condense those to what usually is needed. You go to the service counter of the cellular provider and ask them to fix it. It's rarely your iPad that needs fixing but the account or something on their end.
I have a a IPAD2 with both WiFi and 3G. (16G) I have a contract for 3G usage.
It seems every time I need to use the 3G for internet ,as there is no WiFi service available, I keep getting a message "cannot activate cellular data". I have this turned on as well as "roaming function " from the systems applications.
Can anyone advise what to do? My SIM is correct and I do have a recognized signal from my provider.
Light bulbs you shouldn't buy
There are plenty of dimmable LED light bulbs, but make sure you don't buy the ones that flicker when you dial them down.