From what I understood from the last few days, the funny address you mentioned is an IPv6 address. And when Roon looks at it as the connection address it cannot get back to it. The 192.168.1.100 address is your internal ipv4 address that your internal network is using.
Based on what your ISP was saying, that they don’t support incoming data due to their CGNAT setup. It’s difficult from your perspective as it looks like there is not much you can do without getting more information.