You are certainly grossly overloading the Netgear router's WiFi. There is a limit to the number of devices that can connect "wirelessly" and I suspect you've hit this limit when you start to get conflicts.
Bear in mind also that with WiFi, the available bandwidth, whether it be 54, 150 or 300Mbps, is shared by the devices actively using the feature, unlike "wired ethernet" where each device will have the full 100Mbps speed of the port.
One possible solution would be to use "Homeplugs". Connect the router by ethernet cable to one Homeplug and then locate others where required throughout the house. There are WiFi Homeplugs which would provide additional Wireless Access Points if your connections need to be wireless.