I was very thrilled when the Fritz!Box 4040 was supported by OpenWrt. The Fritz!Box Hardware has frequently been attributed as "solid" in reviews and I have always liked their relatively unobtrusive design. And the 4040 is comparatively cheap.
After having used one of these as an access point without any problems for about a year, I decided to use another one as my central router when my cable provider upgraded to 500/25. This turned out to be a bad decision.
I've created this thread to add some information related to performance. Using all LAN, of course, my provider's speedtest never showed more than 300 Mbit/s. speedof.me produced varying measurements but never more than 130 Mbit/s, which I attributed to a (presumably) bad connection between my provider and the speedof.me servers.
Because of the instability issue, I have replaced the 4040 with a ZyXEL NBG6817. Now I get 480 Mbit/s using my provider's speedtest and 360 Mbit/s using speedof.me.
I don't know if this is related in any way to the instability issue. But at least with 19.07.2 and higher connection bandwidth, the Fritz!Box 4040 is cheap, but not a good bargain.