'Access point temporarily full' on 2.4GHz

I'm having exactly the same problem as described in this post: https://forums.cox.com/forum_home/internet_forum/f/internet-forum/27687/access-point-temporarily-full-on-2-4ghz-is-there-a-limit-of-connected-devices/108616

Unfortunately, that post has been 'locked' (what's the point of that?!) and no solution has been posted.

I have numerous smarthome devices, a good 40 of them, connected to my 2.4GHz wifi network, but there's always at least one that can't connect. When I try to connect my phone, it says 'Access point temporarily full'.

I had pretty much the same set of lights, thermostats, fans and switches at the previous place I lived, and that had no problem. Research shows that wifi networks normally support up to 256 connections at once, and I have nowhere near that number of devices. But my Cox Panoramic access point seems to have some kind of limit, and I can't find a setting to change it. I can connect my phone to the 5GHz network, but not the smart devices; they just don't do that, which is why I turned on a separate 2.4GHz network, as much as the Cox app urged me not to.

Any ideas how to fix this? Some 'advanced' setting hidden somewhere? The Cox app is generally pretty convenient, but it's frustrating the way some settings are so convoluted, probably in a misguided effort to dumb it down.