For a test, does a computer hardwired directly to the C7800 with an ethernet cable have the same problem ?
Not a computer but my ring alarm is hardwired and it drops. I get the notification that the alarm is on cellular backup every time it does it.
What do the modem's signal stats look like ? Try getting them here http://192.168.100.1 or here http://10.0.0.1
Please post the *Downstream Power Level*, the *Upstream Power Level*, and the *SNR* (Signal to Noise Ratio) numbers.
Did you ever find a solution? I am having this exact same issue with the same Netgear C7800 modem/router and internet service! I have my PC hardwired and never experience drops but my Wifi always drops out of nowhere and it is hard to reconnect or connect new devices to either 2.4 or 5ghz.