GCP: Api Gateway failing to redirect to an url made from a private dns

Question

I have a vm instance at IP 35.187.188.140. I created two url representing this service by two different ways, and I inject it in the Gateway configuration file.

http://35.187.188.140.nip.io is a url created by nio.io service. Given an IP, you get a public url http://nik.internal.zone:3000 is a url made from a DNS Zone, with A -> The good IP

The gateway works perfectly with the public nip address, but fails with the private url ginving the message:

{"message":"no healthy upstream","code":503}

However, from a second vm instance, I'm perfectly able to reach the service using the private url !

So here is the full config

swagger: '2.0'
info:
  title: nipornot2
  description: API test for backend
  version: 1.0.0
schemes:
  - https
produces:
  - application/json
paths:
  /api/v1/nip:
    get:
      summary: Greet a User from service
      operationId: hellonip
      x-google-backend:
        address: http://35.187.188.140.nip.io:3000/api/v1/service
      responses:
        '200':
          description: A successful response
          schema:
            type: string
  /api/v1/service:
    get:
      summary: Greet a User from service
      operationId: hello
      x-google-backend:
        address: http://nik.internal.zone:3000/api/v1/service
      responses:
        '200':
          description: A successful response
          schema:
            type: string

Maybe internal url are not allowed for some reason but I don't see a good alternative, maybe I miss something in the config. Any idea ?