You may not need this plugin since serverless/serverless#6127 was merged.
Resolve serverless invalid principal issue when deploying in China region.
$ npm i serverless-plugin-aws-cn-principal -D/* serverless.yml */
# ...
plugins:
  - serverless-plugin-aws-cn-principal
provider:
  name: aws
  region: cn-north-1
  endpointType: regional
# ...$ serverless deploy -v