fritz.box
I do use a Fritzbox too. Unfortunatelly, there are a few chipsets of Wlan-Dongles which are not completly supported. I do own some dongle with a RTL chipset. If I use this dongle I will get disconnected a lot of times.
You should try a wired connection for testing.
Also, if you are using a dongle, you should switch the WLAN channel to a fixed unused channel. It is very important that you disable autochannel. Also set transmition power for Wlan in the Fritzbox to 100% disable auto.
Edit: To be honest with you I did own a dongle with a RTL chipset. I do not own it anymore. When I tried to get some stuff from Ebay I got disconnected and failed. Usually Iam quiet and polite but this time I got so very angry that I took the dongle went to my car and drove over it a couple of times until it became a very slimline dongle. I might have lost a wheel by getting a flat tire but it still did worth it. So do not waste time on unsupported dongles. Edit2: ping for 5 minutes is a waste of time. Leave this pingwindow open untill you get freezers. When freezes appear check ping.