Hi, when recently adding Sonoff R3 and R2 switches, wifi behaved very strange. Several devices, including my phone, suddenly could connect to wifi but had no internet. On the network list in my Fritz!box router a lot of active wifi connections were listed as inactive. Only after resetting my Fritz! mesh access point things were in order again. So my guess is that the Sonoff device registers with the access point and not with the router itself, causing the access point to malfunction. My advice is to add sonoff devices near the router and then move them to the intended location.
I leave it to you to contact AVM (Fritz) if necessary.