Moving from RT-AC86U x 2 to ZenWifi AC (CT8) x 2

Having had 2 x AC68U units working ok I decided to sell them and move to 2 x AC86U routers (better coverage), upgraded to Merlin, put them in Access Point mode/Mesh mode as I did with the old AC86U, both connected via Ethernet cable, and found that the connection to 5GHZ wifi is flaky. Not that the wifi connection drops, its stable, just that I randomly get DNS Probe Internet errors when browsing the web on my laptops and mobile phone devices.

I thought it was just the one unit doing it...

Moving from RT-AC86U x 2 to ZenWifi AC (CT8) x 2