This is the 9772nd most frequent Korean word.
고원이라지만
"고원이라지만" means "though it is said to be a plateau."
고원이라지만 is used to introduce a contrasting statement showing that though the place is identified as a plateau, in reality, it is not necessarily so.
이곳은 고원이라지만 실제로는 평평한 땅입니다.
This place is said to be a plateau, but in reality, it is flat land.
Here, 고원이라지만 serves to describe an expectation based on the characteristic of a plateau, which contrasts with the stated condition of hot weather.
고원이라지만 날씨는 매우 덥습니다.
Although it's said to be a plateau, the weather here is very hot.
The term 고원이라지만 here is used to set a premise (the area being a plateau) before stating an additional positive attribute (beautiful scenery).