Got a new network card, which broke port forwarding. Seemed like an easy fix: I'd set up my static device IP in my Windows network settings, and my port forwarding rule was thus applied to the wrong IP. Changed the IP for the port forwarding rule, used the DHCP Reservation settings in my router to lock my computer to that IP and prevent having to hassle with it in the future...port still not forwarded.
I've checked Windows firewall, and there are rules allowing traffic on the correct port, and rules allowing Transmission to pass through. I'm at a loss as to what else to check here.