7490 (7390) both just work for me.

As for SIP I've set up both my mobile as a Handset to the home number, and another phone as a second VOIP number.

The wireless on the 7490 seems better that the 7390 (both the g is better on the 7490 and the fact the 7490 has ac)

I'm sure they are not as awesome as and edge router but for a free box they are pretty well featured.

I'd try was a previous poster suggested and start with your test PC as the only thing connected to the Fritz (and connected by Ethernet).

Then change one thing at a time until you find the issue.
(In case the first test shows the Fritz as the point of failure ideally test another PC to confirm the fritz causing the same issue on both Machines. Also consider swapping Ethernet cables to eliminate them as the cause - both form ONT to Fritz and Fritz to PC, again separately to isolate issues.)

Yes, I know you have had the cables/ports tested - I probably have trust issues :)

Good luck, It can be no fun troubleshooting these issues