In the Gnarled Root Dungeon, you receive Ember Seeds, which gives you access to the Eastern Suburbs. You can get there from Horon Village.

Go to the Eastern Suburbs

South of the Maku Tree, there is a sleeping guy and a kid playing fetch with his dog. When you approach the two small trees on the right side of this screen, there will be a cutscene. Afterward, burn the small trees and exit to the east. You are now in the Eastern Suburbs. Go north, then north from the screen with the windmill, then east. A strange person rushes past, then goes north.

Go north and follow the person without being seen. When the person goes past the fence, don't follow right away, because the person will come back and look around one more time. After that, go west and you will see the person disappear into the northwest bush. Cut that bush to find a portal to Subrosia.