@David-TP Forgot to mention, the lay out as you have drawn is correct.

@David-TP The switch is a TL-SG1008D Gigabit 8 port switch The DECO S4 is hardware V2.0, Firmware Vsn 1.4.3, Build 20200918, Rel 77820 When I disconnect the ethernet back haul the system does recover

I have 3 x Deco S4, one is the master, one is connected via WiFi and one should be connected via the Ethernet backhaul. the ethernet cable runs from the master to a TP-Link switch where among others...

Struggling with the same problem on my AX6000!!