you talk to a little girl that's by the teleporter inside the red ribbon army base she will teleport you inside the red ribbon base
That girl takes you to the Red Ribbon HQ Ruins, not the Underground one.
The name is UD ( underground dungeons), but the UD1 and others are disabled at the moment.
Oh. Thanks. Wait if they're disabled then you literally can't do most of the quests in Korin Forest and it's just pure grinding at that point?