Slow Gigabit Speeds. Tech came to my house and said the issue is on my end without any testing.

I'm paying for the gigabit service from AT&T. Here's what I've done so far. 

• I've gone through 2 different gateways from AT&T. Both had the same results.
• I've factory reset both gateways and the same results. 
• I made sure IPV6 is turned off.
• I made sure nothing but my 2017 MacBook Pro is connected to the gateway via a CAT5E ethernet cable (tried multiple cables as well)
• Since there's no built-in ethernet port, I have a USB-C to Gigabit ethernet dongle from both Anker and Belkin. Same results. 
• My whole house is wired with CAT5E and all of my switches are gigabit ethernet switches. I tested speeds with my gaming PC (which is on a different floor, but wired to the CAT5E cables running through the gigabit ethernet switches. Same results.

I'm basically getting anywhere from 300-500mbps down and averaging at around 400 most of the time. I've never seen it hit the 500's ever. The AT&T Tech wouldn't do anything for me because he ran the internal test from AT&T to my gateway and it shows gigabit speeds. I've run the same test from AT&T's customer support pages and as you can see in the screenshot below, shows I'm getting a proper speed to my gateway. Not sure how accurate that number is if I don't get anything near those speeds in any real world tests. As you can see from the screenshots below I'm getting between 400-500 mbps from the major speed test sites. Netflix's speed test is always terribly slow for some reason. And all the other speed test sites I've found on the internet tend to side with fast.com. 

What should be my next step at this point? The AT&T Tech came with an iPad only and said he doesn't have the tools to test my connection from an ethernet cable. He blames my USB C dongles for being too slow. I thought USB-C can transfer data with speeds much faster than 1GB? Or he thinks my computer is too slow which is ridiculous.