My usually use ISP router (ZTE GPON) wired connected to Linksys E2700 configured as wifi access point and Linksys WET610n as wifi bridge directly connected to Bartok. Also I tried ISP router to Bartok directly.
I also used Melco between wifi bridge and Bartok
(Major red flag! In fact, I’m surprised you’re not seeing problems regularly 
)
The main problem was just yesterday.
Of course, such connection losses sometimes happen, rarely, but they were with ISP router also.
Yesterday it was cyclical for an hour, in both cases, then I just turned everything off and decided to check the next day. Then I found this topic.