Termini to FCO

There is a very convenient train going from Termini to FCO every 30 min. It’s called the Leonardo Express. I think it costs 14€, just a bit more than the Terravision bus but faster.
www.trenitalia.com
from: ROMA TERMINI
To: FIUMICINO AEROPORTO.

Which hotel and where exactly? If it’s the Hilton at Fiumicino airport you can walk there from the FCO train station using the pedestrian aerial skywalk.

The Sabrina Airport. Main reason is its beach nearby. Thank you for your reply.

The Sabrina Airport seems to be in Focene.
I think the more convenient way is to get a taxi once you are at the airport, because by bus you'd probably need a couple of buses, first to Fiumicino (town) then to Focene.
I'll warn you that taxicabs in Italy aren't cheap, and in spite of the short distance between the airport and Focene, you are likely to pay a fare between 30 and 40 euro.

you are likely to pay a fare between 30 and 40 euro.

That seems pretty steep for a ~ 4.5 mile trip ( 7.0 km per google)

Taxi fares from the airport to anyplace in the municipality of Fiumicino (Focene is part of the Fiumicino municipality) is subject to a minimum fare of €25.

Fiumicino town center is only 2 miles from the terminals, so less than half the distance to the Sabrina Airport hotel..

So, yes. It is steep.

Welcome to the country where the taxi lobby has so much power to basically put Uber out of business.