AAAALLLLRIGHT!
After almost a month I finally got in touch with my ISP's actual IT specialists (the secret was to call them right before 11PM). Turns out that the previous information I'd been given was false and that I'd been behind a CG-NAT this whole time. The lovely IT support person solved everything and this topic is as good as closed.
To anyone who's stumbled upon this forum post via google searches, be sure to ask your ISP to forward you to their expert technical support department as the lower levels of customer support may not be informed enough or have the technical knowledge to assist you with your problem. In my case, the issue was that I was behind a NAT which doesn't allow for port forwarding.