Combine the achiote seeds with 3/4 cup warm water, then set aside.
Place a medium to a large pan over medium-high heat, then pour oil.
Once the oil is ready, add garlic and onion and cook until it softens.
Add celery and bouillon cube, and sauté for a couple of minutes.
Add carrots and cooked chicken, then stir to combine.
Pour about 5 cups of the stock from cooking the chicken earlier.
Strain the achiote seeds and add the reddish liquid as well.
Cover, boil, then simmer for 10 minutes --- taste and adjust the seasoning by adding fish sauce and pepper.
Add the chopped cabbage and sotanghon noodles.
Stir gently to submerge the cabbage and sotanghon in the soup fully.
Cover for a couple of minutes or until the noodles are cooked.
Transfer your chicken sotanghon soup into bowls and serve!