Orbi RBR350 AP Mode issues

Hi everyone.

I am currently dealing with a situation involving 5 Orbi RBR350 Mesh APs.

These devices were configured in router mode until the ISP moved the landline to a digital service which can only be accessed via the ISP router. 

I have altered the Orbis accordingly, they are now operating in AP mode..... Awfully.

For starters, I can't always get the main unit with the ethernet connection to the router to see an IP address, resulting in a purple solid light. Web UI shows no connection. Eventually, this will resolve it's self but drops are little and often.

All the Orbi units are wired together except one on a window sill pointing at the back garden, but this unit is line of sight to a wired unit (the walls in this building are incredibly thick, mesh will NOT work via walls hence the wiring). 

The WiFi is broadcasting in WiFi A mode (No WiFi 6 etc) and the fastest speed achievable is about 20Mbps... often only 15 when the orbis are recieving the full 35Mbps connection - this is tested and working fine at the ISP router.

Any help would be appreciated, especially with the WiFi issue.