You’re from austria, and this is the swiss forum. I can only tell you what’s possible here.
You probably got DS-Lite => private Ipv4 and public Ipv6, so you VPN won’t work with Ipv4.
In Switzerland you can call the support line and tell them you need a public Ipv4 and they will disable DS-Lite. That nearly always solves problems with VPN and strict NAT-Types in Games.
You can give it a try with the hotline and ask for.