AX1600 EAX12 stopped working - how do I fix this ?

All of a sudden this evening, one of my AX1600 EAX12 extenders stopped working.

I have two (for front and rear of building) and I use them primarily to hook up LAN cable connected devices (which don't all have wifi connectivity !) via a netgear switch/hub.

The meshes connect by 5GHz wifi to my ASUS DSL-AC68U modem router, which uses WPA2 wifi authentication plus encryption.

1)  Connecting direct to ASUS DSL-AC68U by LAN cable, with AX1600 EAX12 unplugged, works fine

2)  Connecting by WiFi direct to the ASUS DSL-AC68U, with AX1600 EAX12 unplugged, works fine

PROBLEM

Once the AX1600 EAX12 is plugged in, there is no LAN device connection or internet connection !

When in same room as my modem/router connected by wifi, plugging in the AX1600 EAX12 extender screws everything:   wifi no longer reaches internet, and if I try connecting by LAN cable between the AX1600 EAX12 extender and my laptop, then there is no internet from the laptop.

My other AX1600 EAX12 extender seems also to be doing the same (though that's still upstairs in its usual location).

This has (of course) happened only a month or so after the expiry of Netgear's "complimentary techincal support".

I don't have a clue how to fix these extenders.

Looking at documentation is just totally confusing - there is just too much to read (!)  .... what I do remember is that when I got them, it was a nightmare to set them up, and I needed telephone technical support to talk me through it.

SOLUTION ?

Does anyone have a simple step by step (that's the important bit) guide on how to reconfigure these extenders without having to spend hours reading and trying to follow technical manuals ?

Many thansks in advance