Getting frustrated with this system because ethernet backhaul will not work (and has never worked)..

The AX3000 bundle contains 3 x Deco X50 units, and each unit has 3 x ethernet ports.

Because I only have 3 units, my incoming internet connection is connected to the "Main" unit by cat 5 cable, and the other two X50 units are connected to "Main" by cat 5.

Operation Mode is set to Wi-Fi Router.

There is no network switch involved.

Normally the 2 secondary units are connected to Main by +/- 150 feet of cat 5 cable.(Note they are not daisy chained. Each secondary unit connects directly to Main by 150 feet cat 5)

I have checked these cables with a cable tester and all four pairs are wired correctly.

Thinking the cable length might be a problem I moved the secondary units directly beside the Main unit and connected them with a 3 foot known good cat 5 cable and they will only connect by WiFi. 

Why are they defaulting to Wifi when there's a wired connection available?