2.4Ghz on R7000P dropping out

I have been using this router for about a year now but only recently I am noticing that all the devices on the 2.4Ghz network are disconnecting about once a week. The 5Ghz network stays connected.

As mentioned I bought this router about a year and a half ago. I am on the latest firmware. I connect the wifi router to a AT&T modem in router mode. I believe I have set this up correctly in that the AT&T modem/router is in passthrough mode to avoid any NAT issues. I followed this guide: http://www.dslreports.com/faq/17734

Over the past year or so I have bought and added multiple smart home devices as well as other peripherals for the house. In total I have about 30 devices connected to the router, most on 2Ghz and some on 5Ghz.

However in the past month I'm noticing the following:
- All the devices on the 2.4Ghz network disconnect about twice a week
- If I try and connect a new device (e.g. my phone) to the 2.4Ghz network it fails to connect
- Restarting the router fixes the problem
- No issues with the 5Ghz network at all

What I've tried to solve this:
- Shifted devices between 2G and 5G (this solves the problem but some of my devices are too far away for 5Ghz)
- Turned of Beamforming and Airtime fairness (I have hesitated turning of MU-MIMO as with so many devices this might significantly degrade my network performance)
- Disabled QoS
- Assigned IP addresses to all smart home devices so they aren't constantly searching
- Made sure my firmware is up to date
- Tried disabling the firewall
- Turned off smart connect

Disabling Beamforming and Airtime fairness had the best impact. Previously the devices would disconnect 1-2 times a week. Since disabling over a week ago the devices have disconnected only once.

Additionally, this only started happening a month ago. Unfortunately I can't remember what changed.