I had a similar issue but it was present regardless of the router used, so on Fritzbox, Draytek etc. This temp. loss of signal occurred sporadically. I don't know exactly what's caused it because even though I had a specialist team here following my CEO complaint, they never told me the reason. During the investigation, however, they found the issue with a lightning protection module, which after replacing did improve the connection in that loss of signal didn't occur as often (there's a wee wire in it and it was broken in our case; probably following a lightning strike), but also did some underground work (loose joint?). They also said something about SHINE being present in my case but claimed their equipment wasn't picking it up, though the issue clearly shows on the ISP's system and on my Fritzbox, but then it's Openreach so they'll even claim that pigs have wings.
What I find interesting is that in your case, you have no pilot reference, but it occurs on my spectrum.
Also, what version of Fritzos are you running?