I am having exactly the same issues with a Surf SOHO MK3 running firmware 7.1.0 build 1284 as in this thread - AP One AC Mini and Balance 20 - WAN connection dropping. WAN connections will randomly drop out. The only thing that restores the connection is to cycle the wifi on the device. I am running mostly Apple devices (two MacBook Pros, multiple iPhones, iPads, and iPods. I have two VLANS operating as different SSIDs- 1. for private networking and the other dedicated to IoT devices (Wemo switches/plugs, Echo, Netatmo, and multiple nest cameras). Forcing the IoT VLAN to 2.4 GHz and the private VLAN to 5.0 GHz seems to help somewhat but then my Wemo switches have difficulty connecting.
I am hopeful that this issue will not take 17+ months to fix as the issue in the other thread (and is still on-going).