Port forwarding needs to be setup on your router ... as there are so many brands of routers and each with different setup menus ... I don’t see how MC can automate this process. This needs to be done by user.
Regarding Plex ... I think it will try to use port farwarding ... but if it fails, it will use proxy as alternative ... I am not sure, but I think it forwards traffic to its servers and from there to your mobile devices (reason I believe it so ... I am sitting behind double NAT ... there is no way port farwarding will work in my setup unless I explicitly request my ISP to farward port from their router to my router ... which I have not done ... yet, Plex able to give me access to my media outside my network)