When your teeth smells like poop it's usually because there's some bacteria on your teeth or you could have bacteria in your mouth that causes the tooth or teeth to smell like poop.
In most cases it's poor tooth or oral hygiene that leads to teeth or a tooth smelling like poop and increasing your teeth brushing and mouth gargling can help get rid of that bacteria and make your teeth and mouth smell better.
So you should brush your teeth more often, use dental floss and also use some mouthwash daily to help get rid of the bacteria in your mouth.
Also gargling with apple cider vinegar is another good way to get rid of bacteria in your mouth.
I used to have my own teeth smelling like poop and after gargling with apple cider vinegar and brushing my teeth more often during the day the teeth stopped smelling like poop.
If doing the above does not stop your teeth smelling like poop then you may have a tooth infection or gum infection that needs looked at by a dentist.