Hello!
So basically. This is caused by recent gamepass update.
Gamepasses have got now their own id (just like developer products). Gamepasses made before the update kept their asset ids (from links), but newer haven’t got them. To prompt purchase you need to call PromptPurchaseGamePass with it’s “gamepass” id. Asset ids won’t work (this is probably reason why method 2 displayed error in your case).
Other things such as 9 gamepass limit aren’t removed.