Need help understanding the internet issue in the US.

Hello there! Among libertarians it is often said that there is no such thing as a free-market monopoly. But weren’t internet providers also in free market?

I am not an american myself and all I know about this story is that several big providers divided the country in parts where only one of them is allowed to provide internet service and thus most if not all of them started providing poor service with connection speed worse than in some third-world countries, while people didn’t have any alternative.

If you are from the US or know about this issue, could you explain to me, how could that happen? Are there still no alternatives? I know you could count Starlink as an alternative, but what about more conventional options?