Entrusting our speech to multiple different corporate actors is always risky. Yet given how most of the internet is currently structured, our online expression largely depends on a set of private companies ranging from our direct Internet service providers and platforms, to upstream ISPs (sometimes...
The next step should be law enforcement: they can contact the service, and escalate to the judicial system if it refuses to act, which can decide whether to order the ISP to block the service, or close the company completely, or even jail the people behind it.
The ISP does never need listen to,.or even hear about, a problem with a service.