@JM77, from reading online it appears Swisscom uses CGNAT in their network. CGNAT allows a service provider to share their limited number of public IPv4 addresses across their subscriber base.
Unfortunately Roon ARC does not work on networks using CGNAT. There are two possible options. The first is to see if Swisscom can assign your network address a static IP address, although there may be a cost to do so. The second option is to use Tailscale’s VPN service for using ARC outside your home.