The Marketplace page for White Plume Mountain makes no mention of Dungeon of the Mad Mage, only Tales from the Yawning Portal. Where are you seeing a reference to DotMM? In any case, it is definitely from Yawning, but all of the modules in that expansion are pretty much standalone. There is a paragraph in the full module about how to use the modules as a full campaign, but in reality it boils down to "the characters hear rumors that lead them to the next module". there is no overarching story or theme. The next module level-wise would be Dead in Thay .