@Marvin_S 

UPDATE

I have tried to troubleshoot some more before replying to your post.

I have removed the VX1800v and reinstated the ISP provied router to help eliminate any problems caused by the VX1800v as everything seemed to work without issue before the VX1800v was installed.

Reviewing the AX53 system log I could see that the AX53 was trying to renew its lease every 3-8 seconds. I searched on this forum for similar problems and found that some have resolved this by setting a static IP address.  I set a static IP address but this did not solve the renew lease problem.  After some head scratching I resolved the issue by changing the DHCP server setting on the AX53 from AUTO to OFF.  Now the system log does not report any lease issues.

Tonight I will remove the ISP router and connect the VX1800v and report back in the next few days. I will reset the VX1800v to factory settings and then change the DHCP setting on the VX1800v from AUTO to ON.  Hoping this will resolve my original problem.

UPDATE.

After many hours of trying to troubleshoot this problem with various setups and configurations I have setup an Easymesh network with the VX1800v as the main device and the AX53 as a satellite with ethernet backhaul.  This cured the offline clients and they now show on the list as permanently connected until they are disconnected.  Plus I now get seamless roaming throughout the property.  Although I now do not have the option for an IoT network for my many IoT devices but I am willing to fore go this to keep my sanity!  Hopefully this will become available with a future firmware update but I wont be holding my breath!

However another bug has become apparent....... some 2.4GHz only wi-fi devices connected to the AX53 satellite show on the clients list as connected to 5GHz band.  I am willing to overlook this as a minor bug and get on with my life.