I was able to solve it by changing the subscription on my simcard.
I was using prepaid cards which means that when the 4G-router connects to the provider, it gets a private IP address from the provider.
The public IP-address that you're seeing is actually that of provider and not of your 4G-router.
So I contacted my provider and asked them if I could get a public IP on my sim but for that I had to get a subscription and then they could give me a public IP.
So now it works I and I use noip.com as a dynamic dns server to be able to connect to my system using a fixed url instead of having to check which ip I have