G1100 2.4 wifi slow or impossible to connect

I've had a problem develop with my G1100 in the last 2-3 weeks which has essentially rendered the 2.4ghz wifi interface unusable. 5ghz wifi and ethernet all working fine, I can run a speedtest over 5ghz and ethernet with no LAN or WAN performance or connectivity issues. Here's the weird thing, I also have a guest wifi that also works fine and it appears to share the same 2.4ghz physical interface as the 2.4ghz non-guest network. For example, I can run a simple test that goes like this:

- Disconnect all 2.4ghz clients from both the 2.4ghz Non-Guest and Guest networks
- Connect a single Mac to the Guest network
- Ping the 192.168.1.1 router interface
- Pings run consistent at about 15-40ms (similar to my 5ghz interface). I can also run a speedtest and the results look good - 75/75
- Switch the same Mac to the 2.4ghz Non-Guest wifi
- Ping the 192.168.1.1 router interface
- Pings run at a minimum in the 100ms range but frequently jump to 1000-2000ms and average around 400-500ms! If I log into the router it is extremely slow. Other devices like an iPhone or Nest thermostat sometimes can't even connect to the 2.4ghz wifi, they get prompted for the wifi password, and/or fail to connect.

I'm on my 3rd G1100 after spending hours on the phone with tech support to no avail so I don't think it is a hardware problem (at least specific to a particular router). The G1100 I'm using now is hardware 1.04 which is the latest. Firmware is 02.01.00.05. Given that the Guest and Non-Guest wifi's run on the same 2.4ghz channel I don't believe this is an air interface issue either since Guest seems to run fine.

I've tried changing mode settings on the 2.4ghz network, disable/enable the interface, master reset/config reloaded but nothing seems to bring 2.4ghz out of its funk. The other other suspicious thing I notice is that the receive errors on the 2.4ghz interface steadily climb, only on the 2.4ghz interface, even when there is no 2.4ghz client connected to the network.

I see a number other threads on this group where people have had similar issues but no resolution.

The frustrating thing is that this was working fine up until about 2-3 weeks ago which makes me suspicious that it might have something to do with the Home Network Protection (McAfee) that Verizon released back in Nov/Dec. I've disabled this but I've also had other issues with this new firewall in the router preventing access to certain servers used by Adobe Lightroom. Wondering if it might be playing a role here.

I'm to the point now where I'm suspecting some type of bug or throttling issue in the router.

Suggestions (other than installing a 3rd party router) of other things I could try to diagnose the issue?