R6400v2 2.4ghz network crawling, but 5ghz network fine

In the past week or so, my 2.4 ghz network has been unusable. Many devices are suddenly unable to even connect to the network. Others can connect to the network, but not the internet, and others can connect but get speed limited to at most 1Mbps, even when the device is in the same room as the router.

The 5 ghz network doesn't seem to be having any issues at all. My service plan is 100Mbps down/12Mbps up, and I am able to achieve these speeds on the 5 ghz network just fine, so I know it's not an issue with my ISP. 

The strangest thing is that while my phone is connected to the 2.4 ghz network, the nighthawk app shows that the connection speed to the router is fluctuating anywhere between 1Mbps and 174Mbps over the matter of seconds. 

I have tried restarting the router, and this fixes the issue for all of 5 minutes, and then the problem is back. I am wondering if this is the result of a recent firmware update. Is anybody else having this issue, or does anybody have any idea what might be happening. I'm more than happy to provide any other information that might be helpful in diagnosing the issue. I would love to forget the 2.4ghz network and just rely on the 5ghz network, but certain devices can only connect to the former.