Low speeds on wifi and ethernet for Giga service

Issue

I've had Giga service for a few months and haven't been getting the speeds I expected.  

Speed details

Sites Used (3 total):

1) Google speed test

2) AT&T speed test (error when on Ethernet - screenshot attached)

Error when using ATT speedtest with ethernet connection

3) speedtest.net

Test Parameters

Devices available at home: 6 (2 laptops, 2 phones, 2 tablets)

Device used as load: 1 laptop (no other devices connected)

Notes:

Initially the speeds with Ethernet were about ~400Mbps, no more than ~35Mbps with wifi.  My first contact with tech support clarified that I had the old wifi router still.  They sent me a new one for Giga (Model 5268 AC), which I installed.  Wifi improved to ~50-70Mbps, but Ethernet went down to ~100Mbps.  I contacted Tech support again, and they simply reset the router.  It improved moderately, but I have 6 devices at home.  If they are all going, wifi is never more than 54Mbps, Ethernet is never more than 100Mbps.  

Questions

1) If I need to follow-up with tech support again, what would I need to request in order to actually get something done right?  

2) Could this be device driven (i.e. the devices I'm using can't make use of higher speeds)?  

Any thoughts/insights on what might be the issue would be greatly appreciated.