Not getting any new main quest after allies of Bruma?

[David's Bottom]

I finished the allies of Bruma quest, I got the Great Welkynd stone for Martin, he said I need a Great Sigil stone from a Great Gate, but after the conversation there's no new quests? Am I supposed to find this gate somewhere or what's happening? There's a Sigil artifact beside the Welkynd stone in the temple but I don't get any prompt on it like I do the Welkynd stone