Port opening troubles w/ Synology + Arris BGW210-700

Hi folks:

I know others have had issues with this, but I'm afraid after involving both Synology and AT&T's paid support, I'm no closer to a solution.

I have a Synology DS212j that I'd been successfully and happily connecting to for remote work for years under TWC/Spectrum. Even during non-pandemic times, I work at home and away, so this access is essential to what I do.

Once I switched over to AT&T gigabit fiber, this functionality stopped working. I tried the usual recommended steps, i.e., logging into the Arris control panel and creating an Application Hosting Entry with the relevant device and ports on the Firewall > NAT/Gaming page. This didn't work for me, so I paid AT&T ConnecTech their $49.99 to have someone log in to my system remotely and basically do the same thing. Again, didn't work. They told me that was all they could do and kicked me over to Synology.

Synology asked for a schematic of my network and did their own test using the port checker at http://www.yougetsignal.com/tools/open-ports. This showed that none of the ports on my network are open, despite AT&T's insistence otherwise. As they say: "We wouldn't have anything we could do from the NAS's end to open the ports. The only thing that the NAS can do is block the ports from the NAS itself, via the firewall, that is disabled by default. You will need to contact Arris or AT&T if that still doesn't work, as they have features enabled that are blocking ports."

So I'm back at square one, having paid AT&T for basically nothing. Is there another option here? Would it work to use the Arris as a passthrough to a different router? (Not that I want to spend the money on another piece of kit when this one should work.) Is there another method to access the Synology that I'm not considering?

(Also, I'm not able to open ports on the Arris for torrent activity either, despite going through the same process. Is the NAT/Gaming section actually workable for anything?)