This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
炖 combines 火 (fire) with 屯, which likely contributes the sound. The exact historical development is uncertain.
With fire 火 on the left and 屯 on the right, picture a fire used for slow cooking or stewing, which fits the Chinese meaning of stewing or simmering. In Japanese, the meaning is not established, so treat it as a fire-related kanji of uncertain use.
For トン, imagine a ton of firewood burning under a pot: ton -> トン, and the fire 火 keeps the stew simmering.