This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
璮 combines 王 (jade) with 亶, which likely contributes the sound. The exact historical development is uncertain.
The left side 王 suggests jade or a precious stone, while the right side 亶 is a phonetic component. Without a clear meaning, picture a rare, uncut gem whose name is a mystery.
For タン, imagine a tan-colored jade stone: tan -> タン, and the stone's color helps you remember the reading.