Hey there! 
Thanks so much for your response. Some of it I don't quite understand, unfortunately. So, I hope you may be able to answer my follow-up questions:
- You're saying Vodafone and Cable is the issue, correct? Is that because I'm in a CGNAT? In other words, I share a public IP address with other people? Would internet from Telekom solve this issue? (my sister has that in her home)
- In case I stick to Vodafone, you're saying that Tailscale might be an option? Can you tell me what it depends on whether or not that is an option?
- In that guide you sent, they mention Astrorelay for people who are "in a CGNAT". Is that something similar / the same as Tailscale?
- In case I get a dedicated IP (let's say from Surfshark which I already use) would that significantly simplify this entire process? Can I just always connect to that IP address then with my travel router?
As you can probably tell, I'm quite the noob. So I really appreciate it! 