The new iPad users have noticed that their tablet tends to lose the 3G connection, after it connects to a Wi-Fi network - where there are no visual indications of the problem.
However, the carrier 3G logo is still in its place and the reception bars suggest that everything is in order, but any operation that needs a connection fails. Users have reported that the error message "Could not activate cellular data network" appears.
A possible solution that works in many cases is to reboot the new iPad. Others say that entering the Airplane mode, once or more times, could offer a temporary fix.
Either way, if a new iPad owner uses a Wi-Fi network, the cellular connection becomes erratic again. So far, Apple has not acknowledged the issue that has been reported in US, Spain, UK, France, Switzerland and other countries.
CultOfMac suggests that the cause might be an incompatibility between the new LTE chip and the old 3G SIMs. Until Apple delivers an official bug fix, CultOfMac suggests a three-step workaround: "1. Deactivate SIM PIN (Settings>Cellular Data>SIM PIN; 2. Reboot; 3. Optional but recommended: Re-enable your SIM."
Source: CultOfMac (opens in new tab)