RAX45 shows 5GHz devices connected; will randomly halt connections from devices showing connected

Router: RAX45

Firmware: latest, Firmware Version 1.0.2.72

Modem: AT&T supplied gigabit fiber modem

Devices (5GHz): Several macbooks, iPhone 11 pro max, Sony X900H

I have an RAX45 purchased from Costco which has been nothing but problems from the start. When connected, it works all right; the speeds are decent (~500/500 overwireless, full gigabit over LAN). However, it has this nagging problem that I cannot solve with updating the firmware, changing wireless settings, altering DNS, DHCP manual or automatic, or IP tables, etc.

When I am connected at home, I have many IoT devices which all connect to 2.4Ghz network, which appear to stay perpetually connected. In any case, this is the more stable connection, albeit MUCH slower (70/30). My main devices (laptops, TV, iPhone) are all connected to the 5Ghz network and set to high priority in the device table.

Since I have purchased the router, often throughout the day, I will see my devices show they are connected to the Wifi internet, but will fail to actually connect to websites, services or even the router page (10.0.0.1) for several minutes. This happens seemingly at random and even during crucial actions, which scare me when I need to remain connected to business calls or have files available quickly from cloud storage. It has happened since before I updated the firmware (originally shipped with the .32 firmware), and during each iteration.

I have no idea why this happens, or even how to replicate it. I did notice that when I introduce a new network device on the 5Ghz, it will increase the likelihood of all the other devices -- including the new one -- of exhibiting this behavior.

Does anyone have any similar experience, suggestions, or otherwise help to give? I am at a loss and this router will soon return to Costco if I can't solve it.

Much thanks to all.