Internal Domain not working When communicating from NextJS backend to other service

Hi I have a NEXTJS UI, and another backend service (we'll call this "Scheduling API") that my NextJS backend calls When I had my NextJS Backend talk to the Scheduling API by hosting Scheduling API on a public domain, the communication worked fine. When I switch the scheduling API to be Private Networking only, the communication does not work. My NextJS backend calls the Scheduling API to fetch some data BEFORE the HTML is sent over to the user's browser, so it is not actually the user's browser that is trying to call the Scheduling API Here is a screenshot of my Scheduling API networking configuration: (image 1) Here is a screenshot of the logs of a successful Scheduling API deploy showing that the scheduling API is running on prod on port 8080: (image 2) For debugging purposes: I tried calling the Scheduling API with 15 different permutations of urls, and none of them worked (images 3 and 4) What am I doing wrong? I'd like to not have to publicly expose my Scheduling API to the public internet.