Yes, I should have checked that. It might very well be the RE850 causing the issues. I am unfortunately 350 miles away now but will surely try this next time I am there. The reason why I added the extender in the first place is that a I have Wi-Fi connected fan controller in my greywater sewage...
I have had an Asus XT8 AI mesh in my summer house which worked well for more than year.
I have used both a fibre terminal (ONT) and a TP-Link Archer MR600 LTE router as WAN connections (but not simultaneously). My network configuration...