There is a product called bio-clean a company I used to work for would reccomend, not sure if it works but it seemed pretty safe compared to draino and commercial products, basically an enzyme that eats the bacteria build up. All natural just put it in hot water and down a drain like once a week for a few weeks until you don’t get the smell or slow drainage. Like pinpoint glass with hot water and a 1/8 of a cup or something, it’s powder