Ethernet works/No WiFi Network seen to connect wirelessly

TL;DR: Ethernet plugged in works with computer, no WiFi network observed for wireless service.

Modem/Router:  NETGEAR CG3000D

Okay so I am new to Xfinity and had a tech came over today to activate my service.  I had my own modem, made sure it matched all the requirements.  Wifi works when he connects me, then 5 minutes after he left, Wifi dissappears and I can't locate it in the available networks.  But when plugged in with Ethernet cable, I can get online, but still no wireless. 

I know it's not the modem/router fault since it had worked when the tech was even here, and I brought it over to a friend's house to make sure it works.  Called customer support, complete waste of time, they keep telling me it's the modem fault and I need to contact manufacturer, even though I told them that it works and I know it works.  Nothing now helps, and I can't connect my phone wireless.  I am frustrated and if I can't get this resolve by next Friday, I'm just going to wash my hands and cancel service, but have been looking at all the websites to see what the heck is wrong.