@ayeni 404 error means that the URI that you are calling is incorrect. As you have mentioned that the test from azure portal works fine but when you are testing outside the azure portal (like REST client etc.) you are not able to hit the API and it returns 404 error. Please confirm if my understanding is correct.
Can you please navigate to any of your API that you are testing and in the Test tab please verify the Request URL that you are calling from your REST client is correct or not.
If you are still facing the issue please share the screenshot and error details.