It probably wouldn't hurt to reset the router to factory settings. It's been known to work for other people. It's especially recommended to do this after a firmware upgrade.
Beyond that, assuming that the 2.4 GHz radio hasn't physically burned out, the most likely cause is environmental. Use a Wi-Fi analyzer, like inSSIDer, ViStumbler or Acrylic WiFi, and survey the active Wi-Fi networks around you. Set your router to use the least crowded channel.
Unless you happen to live in an area with no other Wi-Fi networks nearby, I recommend that you stick to 20 MHz wide channels for the 2.4 GHz band. 40 MHz channels consume 2/3 of the available spectrum, so it's not good neighborly netizenship to hog so much spectrum. I've seen some reports that claim that 40 MHz performance can actually be worse than 20 MHz if conditions are bad enough. On the Nighthawk routers, like the R7000, you can disable 40 MHz channels by decreasing the Wireless speed to Up to 289 Mbps.
USB 3.0 is known to generate radio interference in the 2.4 GHz range. While the USB 3.0 port on the R7000 is one of the better shielded ports compared to other brands, using it may still impact Wi-Fi. Temporarily disconnect any device connected to the USB 3.0 port and see if Wi-Fi performance improves. If you are using a USB 3.0 device on a laptop or PC, temporarily disable it, too.
Other sources of noise include microwave ovens (bigtime!), 2.4 GHz cordless phones, and Bluetooth. Try to isolate and eliminate these sources, if possible.
Finally, while Nighthawk routers have some of the strongest radios in the industry, router positioning is important. Ideally, it should be placed within direct line of sight to clients or with as few intervening walls as possible. Make sure the antennas are positioned as recommended by Netgear. On the R7000, this would be the center antenna vertical and the outer antennas at 45 degrees. In a two-story dwelling, place the router on the lower level (unless all of your clients are upstairs!), because the antennas are arrayed to generate radio emissions upward more so than downward. You may also want to experiment with orienting the router so that all antennas are facing the clients as much as possible for maximum signal reception.
Hopefully, these tips will give your clients the best chance to connect and stay connected to your router.
