Did you have 5g, what sim do you have and do you have a 5g broadband contract with 3? If you don’t have a contract but bought a sim elsewhere there may be several reasons why you had 5g and no longer do. 3 may have been testing prior to making it available in your area, you may have been on the edge of reception and 3 have altered something that means you no longer are in range.
More details make it easier for people to postulate a theory.
