I Saved The Cold Lord's Kingdom

Modern girl Sue Lin's bathtub connects to an ancient tub, transferring supplies and letters. Lord Ethan, a border war god, is besieged by Hunnic Tribes with no supplies. Sue Lin sends food, supplies, medicine to help him defend the city. They exchange letters and develop mutual affection. Sue Lin learns Lord Ethan was poisoned to death at 23, so she sends antidote to save him. Bathtub destroyed, they meet across time and space. Trade established, war stops, world peaceful.