CustomID Encoding for GET requests

I have several tables that do not use integers as primary keys, but instead use a 2-4 letter string/code. Currently, making a GET request to the resource throws an error only for the resources that use codes. My tables that do use integers as primary keys work perfectly fine.
GET url/resource/123 returns 200
GET url/resource/ABC returns 404

I see that the laravel JSONAPI docs mention a section on CustomID encoding, but dont offer any explanation for what that means. I am trying to figure out what I need to do to be able to perform a GET request using the string primary key for my resources. I have set the model and uriType attributes correctly in the schema, as well as the primaryKey and keyType attributes in the model.