unfortunately, if you’re aware of its existence and prefer not to use it, there are no (free) alternatives to hamachi with a large userbase, nor are there any (free) alternatives for port forwarding. this is the easiest way to create a non-public server (since it’s basically LAN - actually, VPN, but that’s besides the point)
we have a guide how to set up a hamachi server (without port forwarding) for beamMP on these forums