I have a FP2 about a week now, and since yesterday there are no “available networks” anymore. I tried the SIM in another phone and also a other SIM in the FP2. In other phones the different SIM-cards work in the FP not even one of them is working…
I suppose you have opened the list of available networks in the settings (a window with black background and search button at the bottom) and found none.
Could you try inserting your SIM card in the other slot? The Fairphone 2 has two SIM slots.
Also, in which country are you located and from which operator is the SIM card?