Sorry if you really don't know, sure I will help.
If you buy Premium you can have a domain like yourname.subsonic.org, see subsonic settings -> Network
Once you are premium and have your domain subsonic will send always the new IP to the domain and so Sonos can access always by
http://yourname.subsonic.org:4040 and so on as needed for SONOS.
Since I can't effort Sonos

and I can't Test it, send me an PM and I give you an Test account. Even if I'm Premium, I don't use the subsonic service, I have my own domain, but it is identical to the subsonic service.
There is no need to enter the IP of subsonic. An example
https://google.de is the same like
https://216.58.205.227 hope that is clear
