HG659b issues when used in conjunction with an Access Point

I have a Spark supplied HG659b that works perfectly on Ethernet and Wifi (both 2.4 and 5).

However when I connect (via a LAN cable) an Access Point I get unreliable connection to the internet via the Access Point (the AP even says it has no internet connection when this issue occurs) and we get different devices unable to connect via wifi on both devices (ie including the HG659b).

I have tried 3 different Access Point devices and all have the same issue so I have concluded the problem is in the HG659b setup and it something to do with how it is dealing with IP addresses as it all works perfectly fine when both devices are booted up, but then at a later time these various issues arise. It seems to be most noticeable that it occurs after a device has successfully been using the AP, but then when there is no internet traffic via the AP it is as though the HG659b is disconnecting the internet from the AP.

I have no idea why even devices connected to the HG659b via wifi will lose the connection while others will remain connected. It seems like some sort of IP conflict, but I cannot see why it occurs. However as soon as I switch off the AP the devices immediately get connection to the HG659b again.

The three Access Points I have tried are:

WD MY Net N900 Central operating in AP mode (it has an option to select this)

An Old Vodafone Station with DHCP turned off

An old Vodafone HG 556a with HDCP turned off.

In all cases (and for both the HG659b and the AP) the wifi channels are set to fixed and different channels.

Where possible I have tried the AP using a Static IP.

Does anyone have any suggestions as to what I am missing.

It seems like there must be some setting in the HG659b that I need to change, but I can't think what.

Thanks