In the city name 东莞 the character is read 莞 (third tone), not wǎn or guān. Mispronouncing Dongguan as 'Dōngwǎn' is a very common mistake even among native speakers.
The grass radical 艹 on top. Its original meaning was a marsh sedge; the 'smile' and place-name uses are borrowed sounds, so here the plant radical is historical rather than descriptive.
Below the cap, 完 supplies the sound. Its reading shifts across the three pronunciations of this character (wǎn / guǎn / guān), so the phonetic only loosely fixes the sound.
