Weird issue with new Orbi RBK753, Wi-Fi works fine but loses internet access

I  bought an Orbi RBK753S which worked fine for a couple of months but suddenly was getting all sorts of issues connecting to the internet, Wi-Fi connectivity was fine but I would lose internet access on a number of devices (LG Smart TV, Dell XPS13 laptop, my QNAP NAS servers all lost internet access one by one, not simultaneously). A Orbi reboot would temporarily resolve the situation, but only for a day or two and the issues would pop up again.  I contacted Netgear support and they were of no help. They said if a reboot fixes it, albeit temporarily, that is all  they could do, which I found to be completely unsatisfactory. I cannot be rebooting the Orbi every day or two.

I replaced the RBK753S with a new RBK763S a couple of days ago.  I am now getting other issues with the RBK763S.

My Configuration:

• Orbi RBK763S, router with two satellites running firmware V6.3.1.0_1.1.41
• Arris SB6190 Cable modem (planning to replace with a new modem this week as I understand this one is buggy)
• ISP is Spectrum 200 mb/s
• Primary PC is an AMD 5950X desktop with 32GB RAM connected via LAN port
• Two QNAP NAS servers connected via other two LAN ports
• Number of devices connected to Orbi, approximately 30, but only four or five simultaneously

I have configured the Orbi to use access control. Any device not on the whitelist cannot connect to the Wi-Fi.  The problem is various devices (Apple iPhones, iPads) would drop connection to Wi-Fi.  My wife's HP Spectre X360 laptop connects to Wi-Fi but says there is no access to the internet.  I tried toggling Private Wi-Fi Address on my iPhone wireless settings but that did not work,

If I disable access control, everything worked fine, at least for now (it's only been 12 hours since I disabled access control).

I tried checking my CTS setting but cannot find it anywhere in the web GUI. 

Any assistance would be appreciated.  TIA.