Connecting to API Gateway Custom Domain

I have an API set up with a custom domain name in API Gateway as "api.<exampledomain>.net". This same domain has a Simple Routing entry for "A" to the same ARN for the API. The API is working when testing in the API Gateway. I have invoked the URL directly, which returns the "Missing Authorization" token for both the actual Invoke URL as well as the custom domain above. I am assuming this means the routing is working. I am able to successfully call the API from Postman using the Invoke URL. However, when attempting to invoke the custom domain name using Postman, I get "Mixed Content Error: The request has been blocked because it requested an insecure HTTP resource". I am looking to complete the setup of my dev environment but unable to resolve this last issue. As an aside, I also have a Simple Routing in Route 53 setup to route to an S3 bucket for a static site at <exampledomain>.net and this is working correctly. I have configured IAM and I have a valid certificate issued from Certificate Manager. I only have one routing set up for any domain across my platform. I feel that I have missed a step here.