Please try using both 2.4GHz and 5GHz networks to isolate the issue. Also, make sure that the modem is placed in proper position. The modem should be placed in a open space, 4ft above the ground level. 

Respectfully, I did that. I've spent hours on chat with both Arris reps and Xfinity reps and all of them said everything should be working.

I can't get a straight answer from Arris aside from the first rep suggesting the unit was defective, then the second one had the same problem. Even my old Netgear gateway (which was several years old and the cheapest model at the time) performed significantly better before the antenna broke than this brand new unit, which is why I'm asking if that's normal for the SBG10 or if I truly am so unlucky that I got a defective unit and the replacement was equally defective.

SBG10 compatible speed with Xfinity is 400mbps. Could you let me know the speed plan opted with internet service provider.

If you are experiencing with intermittent internet connection, let us know that the issue is with wired connection or with wireless connection.

Oh, it's not an intermittent Internet connection... The signal doesn't reach 30 feet away. my TV that's barely 20 feet away and a straight shot down the hallway doesn't consistently get enough of a 5G signal from the gateway to stream a regular quality show while the (significantly older) TV in the same room as the gateway has no problem streaming a 4K show.

If you work for Arris, tell me if that's normal or if I really am just incredibly unlucky to get two equally malfunctioning  units right after each other. Because of Arris' replacement policy, I now have a $100+ paper weight that they won't replace or let me return, plus they drew out the troubleshooting time so much that i can't return it to Amazon. Am I just really that unlucky that they screwed me over that hard on their own malfunctioning product?

Usually, 2.4Ghz Wi-Fi network coverage you can expect only up to 20 to 25 feet's maximum and for the 5Ghz network coverage you can expect only up to 10 to 15feets.

Also make sure that there are no interruptions like brick wall, etc.