Salt is IPv6 oriented. I had a Salt Fiber when I was an expat in Basel for 4 years.
I received a private IPv4 address (10.12.x.x) on the public interface of the router. If that’s still the case , it will never work. There are two solutions I can think of: either you pay approx 10 CHF/month to Salt for a publicly accessible IPv4, or you define an IPv6 VPN solution that translates to IPV4 on the core and client.