Built-in xfinity router reduces speed by over 90 percent

I'm paying for 500 Mbps internet.

Modem speed tests regularly report at 118 percent of plan.

I have continually complained of poor wifi signal to my patio TV with repeated buffering issues, going on for over a year now.

I was updated from the black box to the white one. No joy.

The TV is in direct line of sight to the box 15 feet away, with only a glass door between.

Then I was given a POD, placed 6 feet from TV, inside glass door. 

Now it's Worse! 

I'm not just in a buffering hell, now I'm just flat-out kicked off Apps every time it buffers, or just plain denied entry after 30 seconds of trying to open.  

This affects not just my Roku Smart TV on the patio, but also my Roku Express in the living room directly below the Gateway 6 feet above and 9 feet from the POD, and the Roku in my bedroom.  

The living room Roku speed test is always less than 50 Mbps, yet the Gateway is just shy of 600 Mbps. That's over a 90 percent drop in speed.  

My patio TV rarely achieves 35 Mbps.  

Even in the middle of the night this occurs. It's now 7am EST and I'm just now finishing an hour long show I started trying to watch at 4:30am. I was kicked out of the app 8 times in one hour!

The best chance I have to watch streaming TV requires me to disconnect everything else from the network.  

I chose this plan because it said it was for 8-11 networked items, yet it can't even provide reliable wifi for a single TV.  

I recently bought a camera doorbell and camera flood light due to recently having a car vandalized, but haven't even tried to install them yet since a single TV seems to be so troublesome.  

Did I get a defective unit? Or is this SOP for Xfinity?