I recently set up a new modem, and upon doing so, I lost my ability to connect to certain Mumble servers. Not all of them, but certain ones, such as one my friend runs. I've attempted to set up port forwarding, using the port associated with my friend's server, but this does not resolve the issue. Is there a certain port that I should use for forwarding? Is it IPv4 or IPv6? TCP or UCP or both? Any suggestions would help. Thanks!
Generally, I don't think it should be necessary to forward ports on the client side. Only server side.
Because your client initiates the connection, to the servers listening port on TCP. Routers generally allow response packets after a client initiated a connection. The client TCP port is (probably?) random. Same for UDP. The server UDP port is probably the same.
Even if UDP does not work, all the control happens over TCP, like connecting, switching channels, sending text, etc. And if UDP fails Mumble falls back to tunnel the voice data over TCP (which is slower but still works).
Mumble supports both IPv4 and IPv6. It does not matter to it. It is a question of what interfaces the server listens on and your client can use.
What do you mean by setup a modem? A router in front of your local home network?