If you want the vpn way, after the windows explorer and port forward ways are no longer working for you, just download something called Radmin VPN. There is also Hamachi and Zerotier but, Hamachi is no longer supported and Zerotier is hard to configure (Zerotier has high chance to make hosting and joins work). Just know on VPNs only the friends can join and no else public user can.
Install it. After, make a network
Define your network. As radmin vpn, it must obtain password for the network where your friends will join in
Now make the network. Now make your friends join this network with the password
After that, you or any of your friends can host... But as you, just copy your own VPN IP Address (leak it if you want, no matter) and give it to your friends
Make your friends join your SRB2 server as long as they are connected to this VPN room, with that VPN IP and they must be able to join!
Yes yes... only con is that you only need to invite people to play with you, and this make public players not having the ability to see or join your SRB2 game since they need to join your VPN room! I'm really afraid if only friends to play is purely enough... ever since you need more players!