Jamieson, Fausset and Brown on Jeremiah 30:9

9. Instead of serving strangers (Jr 30:8), they shall serve the Lord, their rightful King in the theocracy (Eze 21:27).

David, their king--No king of David's seed has held the scepter since the captivity; for Zerubbabel, though of David's line, never claimed the title of "king." The Son of David, Messiah, must therefore be meant; so the Targum (compare Is 55:3, 4; Eze 34:23, 24; 37:24; Ho 3:5; Ro 11:25-32). He was appointed to the throne of David (Is 9:7; Lu 1:32). He is here joined with Jehovah as claiming equal allegiance. God is our "King," only when we are subject to Christ; God rules us not immediately, but through His Son (Joh 5:22, 23, 27).

raise up--applied to the judges whom God raised up as deliverers of Israel out of the hand of its oppressors (Jud 2:16; 3:9). So Christ was raised up as the antitypical Deliverer (Psa 2:6; Lu 1:69; Ac 2:30; 13:23).