macosx/TransmissionHelp/html/pfrouter.html
If you are using a router, it is probably OK to disable the macOS firewall, as you are already being protected by the router. To disable the firewall, open System Prefs → Security → Firewall. Click Stop.
To forward a port in your router manually:
For more comprehensive instructions specific to your router, click here and choose your router from the list.
If you find yourself having to change your router settings all the time, a static IP address might be beneficial.
A static IP is when your computer is assigned an IP address which does not change. This can be helpful if you have a laptop or have multiple computers on your network. While it makes things a bit simpler, you don't have to have a static IP for BitTorrent or Port Forwarding to work.
A dynamic IP address assigned by your wireless router for example will most likely not change unless you reboot your computer, or leave and rejoin the network. Thus, having a static IP isn't really necessary, especially if your router supports UPnP.