I got myself a BE65 3 unit mesh system to replace my aging single Wifi access point.   I have an Opnsense router that I need to continue using for various reasons, one being that the Deco is nowhere near configurable enough to be my main router.  
The first thing i did after setting it up was turn off DHCP and put it in Access Point mode, so far so good.    Several hours later of nothing 2.4Ghz being able to connect I tried putting them on a guest network and found that ANY 2.4Ghz network is entirely unusable and unstable in Access Point mode.  2 Mbps on at 1Gbps connection.
I then changed the mode to Wi-Fi Router and suddenly 2.4Ghz is usable and actually fantastic, however it is entirely impossible despite many hours of frigging around to route from the wifi network that it creates to my LAN network.    
I then got it to establish a OpenVPN link to a new interface on the main router, however no matter what I tried the Deco never sent packets back over the VPN link to the router. 
If anyone  has a bright idea I havent't thought of let me know before I punt this thing back through the shop window it came from.