First of all, I must say that when 3G signal gets lost and turn into 2G (the "o" icon) IT REFUSES TO GET BACK INTO 3G, NO MATTER WHERE I AM, EVEN IN A STRONG 3G AREA!
Experimenting with turning off and back on several device settings (yes, unfortunately, we resort to that, since all else, including the last 5.1.1 firmware, has failed, just to use our device), I found out that in order to have (at least for a while) a 3G signal, I must do as follows:
1) turn off automatic search of the carrier (mine is Vodafone, Greece)... I must select it manually, otherwise any other action is futile, and
2) regularly (when the 3G signal gets lost) turn off the cellular data and watch the carrier icon until all the bars completely disappear, THEN TURN BACK ON the "cellular data" setting.
ONLY THIS WAY, finding 3G signal is feasible.
I address to APPLE engineers to read this post.. it might be proved useful in order to FIX this bug.
THANK YOU.