@gvanlokven based on the screenshot of the “Here you can see the current internet connectivity status“, it appears Roon Core is behind a Carrier Grade NAT so your ISP will not forward connections from the Internet into your home network. ARC needs port forwarding to work in order to go from your phone + ARC → cellular network → Internet → ISP → home router → Roon Core/Server.
There is a solution many Roon community members are using who are in a similar situation with CgNAT. Please see the information below.
This probably raises more questions and issues for you, but I hope having an option is helpful.