Frontier Airlines IAH Terminal is Terminal A. This terminal is located on the east side of the airport and is connected to the other terminals by a skytrain. Terminal A has two levels: the check-in and departures level on the second floor, and the arrivals level on the first floor. Frontier Airlines’ check-in counters are located on the second floor, near the entrance to the skytrain.
Source: https://www.airfleetrating.com/airport/frontier-terminal-iah-airport/