Thank you for your detailed response.
If you’ve configured a matching port assignment in Roon and the router’s web administration (one of the reserved ports for forwarding per your ISP’s instructions, for example) and still receive a 504 ETIMEDOUT diagnostic message, then it’s likely this CG-NAT is simply innavigable.
It sounds like your ISP won’t offer your account a routable IPv4 address, likely due to an IPv4 shortage in your region.
In that case, until the team implements different methods for connecting outside the home (we’re working on it), you’ll have to pursue a VPN or other unofficial workaround in our #tinkering section or await your ISP’s response. Please see our tracking thread for CG-NAT issues here for updates: ISPs and Routers: List of Known Solutions and Workarounds