Having a weird issue with my AC4000 router. It's been working fine since I first installed it several months ago but then yesterday I started having internet connectivity issues with various devices. All devices are still able to connect to the router fine but random devices have no internet connection. There doesn't seem to be any logical or consistent pattern as to which devices don't have internet. I've tried power cycling and rebooting the modem multiple times and sometimes some devices work fine and others don't, and then I'll reset the router again and the devices that wouldn't work previously now connect to the internet fine and devices that were fine before now won't connect. For example, my phone will always connect to the router without issue but it won't have internet. I'll reset the router and then my phone will have internet but then my wife's phone which was previously fine now won't have internet. It's also happening with various Google devices and laptops. 

I haven't added any new devices recently or made any changes to the router settings since it was installed. The firmware is updated to the current version. I'm at a complete loss as to how to fix this issue or if anyone has experienced this before.