In fact there are little mouth tapes you can buy off amazon to train your mouth closed
Sleep Strips Advanced Gentle Mouth Tape to Prevent Mouth Breathing 60Pcs, Improved Nighttime Sleeping, Less Mouth Breathing, Instant Snoring Relief (2 Month Supply) https://www.amazon.com/dp/B07YFS9YN7/ref=cm_sw_r_cp_api_i_-p3nEb4VZGAP4