Thermos Needs Cleaning
Boiling water and baking powder. Fill with boiling water, add a teaspoon baking powder. It goes all fizzy and when it stops being fizzy, pour out and wash up normally. It doesn’t magically remove all tea stains without a trace, but it lifts them from the surface of the vessel so that they’re easily washed off. If it’s really bad, repeat this process a few times.
Give it a treatment for limescale as well. I tend to go with boiling water and a mild acid there. Wash and rinse well before use.