Lumia 930 losing network signal

[Eugenio Di Tullio]

Hi everyone! I received my Lumia 930 this week and at the beginning I used it only on wifi since I had to change my operator SIM from micro to nano. Everything was great and I was very satisfied of the new toy. Yesterday I finally had the time to get the nano SIM but since than I've noticed I lose the network signal very, very often. One moment I have 5 bar, and the next moment I have no signal at all. The only way to fix it is activating and deactivating the plane mode.
I don't need to say that this is really annoying. What I'd like to know is if you guys already know something about this kind of issue. May it be my operator? May it be the phone? May it be some settings?
I also noticed that even though my SIM is 4G ready, I've never been connected to a 4G network (I live in Lisbon so yes, there is 4G coverage).
Also I hadn't the possibility to use another SIM to make some checks yet, but I soon will.

Thank you a lot!
Cheers