R. Herzog on One or Two Days Yom Tov in the Southern Negev (23 Tammuz)

שו"ת היכל יצחק אורח חיים סימן נה 23 Tammuz 5709 (1949)

Would a city in the southern Negev such as Eilat keep one or two days of Yom Tov? Is it halachically considered part of the land of Israel?