Ridiculously slow download speeds

I am so frustrated.  We got rid of our 2+ year old rented Xfinity gateway and bought a Netgear C7000v2 gateway.  We upped our Xfinity service to 300 mbps and are getting paralizing speed results on 5G even with only one or two devices connected.  

If we're right on top of the gateway, we are getting around 200 mbps, but if we walk to the other side of the room in a wide open space, we drop to 60 mbps.

I tried working with chat support from Xfinity, and they told me they were getting test results suggesting there was some pole issue, so they sent out a technician.  The technician told us he couldn't find anything wrong with the service, and that he didn't know how to solve issues with Netgear.  After 10 minutes of running tests, he spent the next 20 minutes trying to convince us to go back to a rented gateway.  He said the only way Comcast would guarantee the speeds is if we had their equipment; otherwise, we were out of luck.

Really??  I'm at my wit's end.  I'm not that tech saavy, but I can read/learn/follow directions.  If anyone has a suggestion that might help, I'm all ears. 

Thanks for listening to the rant!