I see you are using the RingCentral .NET SDK. To authenticate with the production credentials, change the last param to true.
RestClient rcsdk = new RestClient(PROD_CLIENTID, PROD_CLIENTSECRET, true);
Is this a free tier app?
The last thing now I can guess is being a free tier app.
As per my conversation with the backend dev support team was when I was facing similar kind of issues not able to test the app in production.
What they said was free tier app will not be able to test it using our account and any other RC user (paid user) can test the integration app in production.
Now I hope this is not the case with you.
We have the same issue: cannot login users with the account details configured for production environment. What is the solution for this issue?
Using the RingCentral Phone for Desktop, you can dial or receive test calls, send and receive test SMS or Fax messages in your sandbox environment.
Download RingCentral Phone for Desktop:
Tip: switch to the "sandbox mode" before logging in the app:
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