Well, you should be able to join games without forwarded ports. This is only needed for hosting a game. So this sounds strange to me, but I already heared about this loading stop at 5% from somebody else. Maybe it is a crucial bug in the actual version?!
But I cant help you with that, I do not understand a thing about this development stuff.

How to forward ports depends on your hardware, especcially on your router.
But for forwarding the ports you have to know your router password (Maybe it is still the standard password which you should find in the manual (or by searching the Internet).
The standard way then should be like this:
1) find out the standard gateway using ipconfig in the command console.
2)type this standard gateway number stuff in the address bar in your internet browser(without any www. or whatever)
3)you should find yourself in some kind of configuration software of your Router (maybe you need the password here)
4)search for a menu which is called "internet" or "port forwarding" or something like that.
5)there you should add the TCP and UDP and the Reference (=TCP) and the Discovery(?) Port (=UDP) Ports which you can find in the Openclonk menu Options/Network

For more detailed information you should provide the brand and the specific model of your router. Or check the Internet. There are lots of tutorials how to forward the ports.

ah, btw about the 5% thing:
Maybe the round had already started. Joining a running game is not possible (yet?). Then it stops at 5% as well, and after some moments you get some message, that joining was not possible or whatever.