Slow internet when connected to Wifi

OK. I think I have solved the problem. Here is what I did:

1. I got rid of the AT&T Gateway. It's now just the Nighthawk connected to the AT&T ONT via a switch in the middle. Instructions here if anyone is interested.
2. After getting rid of the gateway, I factory reset the router again. I set the SSIDs up again, changed the 2.4GHz and 5GHz channels from "Auto" and whatever the default 5GHz channel was to "06" and "161" (looking at a wifi analyzer, revealed that channel 6 was the least congested in my area for 2.4GHz. Channel 161 was not used at all in the 5GHz spectrum).

After doing both of these things, I am now getting between 350Mbps and 525Mbps on the 5GHz network.

I had done step 2, with the gateway in place, several times before. I think the combination of the two together is what solved my issue. 

Thank you everyone for the help.