@gn0s1s I'm pretty sure I've figured out that this is due to WAN auto-sensing. Here's a relevant thread, with no solution from TP-Link (shame on you!). 

@David-TP Yes, I live in a country where Internet access suffers occasional outages, sometimes more frequently.

Connecting the LAN servers to the satellite Deco units (1) is not feasible because the LAN is in the room where the fiber connection comes in, and thus is the room where the main Deco unit resides, and (2) makes no sense as the primary purpose of the LAN being wired is so that it can use as much of the bandwidth as possible, without being limited to WiFi. If I connect it to a satellite, I’m limiting the entire LAN to the WiFi connection between the satellite and the main Deco unit. And if the satellite is wired, then I will run into the same problem with the “smart” auto-sensing, won’t I.

While I understand what you mean when you say “WAN/LAN auto-sensing is not a problem”, please try to understand what your customer hears when you say that, particularly when it CLEARLY IS a problem for that customer as it knocks off his entire LAN. Other routers don’t do this, this is not ordinary behaviour, and it IS a problem for the customer. As such, you SHOULD consider it a problem for TP-Link as well.