I found, think so, a bug.
When you goes to:
System Settings --> More.. --> Mobile networks --> Use only 2G networks
When you change this settings there is up to 1 minut lag and you cannot use 2/3G network, have to wait. Shouldn't be like that. I'm using Toggle2G and is realy frustrating for me.
BTW: I recommend to use Toggle2G because battery runs for much longer time if you use 3G networks.
When you turn off the phone screen, Toggle2g will automatically switch to the 2G, which conserves battery power, when you turn the screen on Toggle2g will returns back(automatically or manualy) to the 3G network.