I’ve used a Fritzbox 7390 with out BT Infinity broadband for a number of years now – getting around 80 Mbit/s down and 20 Mbit/s up – and it’s been generally very stable.
Late in 2015 the upload speed suddenly dropped to 5 Mbit/s – and continued like this until yesterday (19th february) when it suddenly lost the connection to the Internet. investigating this the Fritzbox showed DSL connectivity at the same speed as before (80/5) but no Internet connectivity, so I called BT – which reacted as they normally do, and could not give me much help as I had a non-standard modem/router combination.
I gave in and found the original BT router/modem combo and installed this – and lo and behold everything suddenly worked.
Anything I did could not get the Fritzbox working except for short periods of time.
I found a lot of chatter on the Fritzbox forums about the original drop in upload speed, linking this back to changes in BT’s central switches, but nothing about the newest problem.
I will continue to investigate.