I am constantly having to restart the AP802 (in extender mode) to reconnect with the 2865ax router, the connection is good once it starts, but ceases to work correctly after around 24 hours, either devices cannot connect to either 2.4GHz or 5GHz channels, or they can connect but fail to get an IP address.

I notice the lease time of the router LAN setting is 24 hours, but I would expect the AP to keep getting new IP addresses when it needed to.

Any advice how I could get the AP to work continuously?
 

Try to change the 2.5 and 5 Ghz modes into b/g/n only.

Do you mean b,g, or n only, or b/g/n (which it is already set to)

On the router wireless: b/g/n: thus mode mixed 11b + 11g +11n , as the AP802 are b/g/n only. Sorry for the confusion.