Internet Speeds Slow During Peak Hours

I currently am paying for the 800Mbps plan but hardly ever see those types of speeds. Starting about a month ago, from the times of 6p.m. to 12a.m. the internet speeds are typically a third of the advertised speed. Anytime before or after that the speed is fine. Over the past couple days, the internet speed seems to be diminished most times during the day (even before 6p.m.) up until around 12a.m. I have visited with my local store and they gave me the upgraded gateway system but am still having the same issue. 

The router IS located in the basement, which I understand will potentially cause lower WiFi speeds. However, this problem is still noticeable even when hardwired via an ethernet cable. It is mostly noticeable with gaming systems as the connection is almost not serviceable (connected via ethernet). Also, although being located in the basement, WiFi connection around the rest of the house has been a non issue up until this problem arose recently. 

I have contacted support over the phone but they were hardly useful as they continued to suggest getting the WiFi extenders even thought the problem persists when being connected via ethernet to the main gateway. The store has told me there is little to no other options and sending a tech out is not something that I or the store expects to resolve the issue, that there is simply a connection issue, not a speed problem. 

Not sure where to go from here, with little to know resolutions available what can I do moving forward or who can I contact to get things resolved? Xfinity has been our provider for years without this problem and has been persistent for the better part of a month, everyday. What are my options?