Cleaning the plastic including the pouring edge of the thermos.
Use green soap on the soft side of a sponge, rub rim, leave for a while (about 30 minutes or longer), rinse well, if you do this regularly the thermos is still very nice and clean.
Volcano with boiling water, dishwasher tablet in it to stand for an hour. throw empty and after rinsing, possibly even with a brush. Shiny clean.
Do you want the inside of your thermos have again nice clean? A few spoonfuls of soda do it, fill it with hot water and the next day all the lime / coffee stains disappeared.