I had connectivity hiccups between my FP3 and my Fritzbox 7490 with different symptoms for a while now, especially when using the 2,4 GHz range. My Fritzbox has the latest software update.
Sometimes my network couldn’t be found, then it took ages to establish a connection, the connection could be stable for 30 minutes and suddenly got interrupted and so on. Also when using flight mode my 2.4 GHz Wifi network could never be found but showed up when deactivationg flight mode. Strange.
Other devices had no problems at all using the 2.4 GHz range.
I don’t know if it has already been mentioned in this forum, so excuse me writing down some information everybody knows but me 
Today I had a look at the 2.4 GHz Wifi settings of my Fritzbox and stumbled upon the auto channel setting. I changed it to channel 8 since that was not so overcrowded as the others.
The first thing I noticed was that my FP3 immediately logged into my 2.4 GHz Wifi network after the Fritzbox had finished rebooting. I used it for some surfing and email and found it rock stable. So I returned to the Fritzbox settings and changed it back to auto channel and noticed a setting to provide channels 12 to 13 that was activated. That draw my attentions since not all channels are allowed or tolerated in all countries and maybe these conditions are dependend on the language selection or location settings.
To make it short - I let auto channel active and deactivated channel 12 and 13 using the corresponding option.
All of my initial Wifi problems have gone now.