C7800 - Cannot connect to 2.4Ghz anymore.

One reason it may not be fix is that there maybe a HW limitation on how many devices you can connect to one radio at a time. You can't just connect unlimited amounts of devices to one WiFi radio. There are some limtations. Not all wifi can handle large qualities of devices. Been like this for years. Nature of the beast. 

One thing you might do is connect a wifi extender, like you did, this helps off load some of the wifi devices to a different wifi source to share the band width.