Nighthawk c7800 - internet sluggish & constantly sticks

We are sincerely frustrated because I bought what appeared to be the top of the line modem/router gateway (NightHawak: C7800/x4s-AC3200) only to experience connectivity problems Ever since We installed it!! Sadly I didn’t get it installed for a couple months, and once we did I worked with Amazon & Xfinity over the issues, because the router was supposed to help everything run bettter, but it’s worse! And I had no clue that Netgear only allows 90 days to assist their customers before they charge more money. I am not a technical person, so reading through topics had been like trying to read a whole other language. Will some one chat with me or offer me so help. Please!