GSM networks (SIM PIN problem)
It seems that the 3-21 version does not work with SIM cards (GSM/WCDMA networks) that have a security PIN enabled - the PIN is not requested and the phone cannot log in to operator network.
If the SIM PIN is disabled the phone connects to the network, and even sets the APN correctly.
On the first boot I had a problem with "process com.android.phone has stopped". Then I ran "fix permissions" from the preinstalled ROM Manager (via Clockwork recovery should also work) and rebooted the phone,
AND followed instructions from the first post (try several times if the error throws you out of the network menu):
The best way to switch the network type is to enable airplane mode (via Settings menu, it didn't work via power key!)
head to the Testing screen: open the dialer, and dial *#*#4636#*#* (dial via onscreen keys)
select first menu option, scroll down and switch to CDMA auto (PRL),
then go to Settings, disable airplane mode, and choose WCDMA preferred (or GSM or whatever GSM variant you prefer) in the Mobile networks settings (not in the Testing menu) under Settings -> (Click More) -> Mobile Networks -> Network Mode -> Then your Network Type."
Try several times if you get the "process com.android.phone has stopped" error!
EDIT: GSM/WCDMA data does not work with 3-21. The E or 3G symbol appears for a few seconds, and then disappears. APN is set automatically and cannot be edited.