If you go to other guilds after level 10 hoping to unlock other classes but don't see any quests available, it can only be because you haven't done the MRD class quests offered every 5 levels at the MRDs guild. To be more specific, you must complete the level 10 MRD class quest. This has nothing to do with story progress or running dungeons.

I know you say you've done the MRD quests, but make sure you're not confusing quests offered while playing as MRD with MRD class quests. Check back at the MRDs guild and see if any quests are available.