Jephthah is rebuking the king of the Ammonites with a rhetorical question. This question can be translated as a statement. Alternate translation: "You should only take over the land that Chemosh, your god, gives you." (See: [[rc://en/ta/man/jit/figs-rquestion]])
This is an idiom which means to take control of something. Alternate translation: "take control of" or "take possession of" (See: [[rc://en/ta/man/jit/figs-idiom]])