This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
珋 combines 王 (jade) with 卯, which likely contributes the sound. The exact historical development is uncertain.
The left side 王 suggests jade or a precious stone, and the right side 卯 looks like a sprout or a pair of doors. Picture a rare jade artifact with a sprout-like carving, but its true meaning is unclear.
For リュウ, imagine a rare jewel (王) being pulled through a narrow opening (卯) with a smooth 'ryuu' sound: ryuu -> リュウ.