EB url renders ssl certificate unsafe

I have posted this issue in SO here: https://stackoverflow.com/questions/69207726/aws-eb-significant-issues-getting-ssl-to-work-on-re-terminated-https-behind-ap/69219036#69219036

I will replicate the core information here:

1. I have:

• a parent domain outside AWS
• a subdomain inside AWS
• hosted zones created for the parent domain
• NS records added to parent domain host, as subdomain delegation
• Alias records in my hosted zone which points to my eb environment. I've also tried pointing this to the application load balancer.
• An SSL from ACM on the load balancer

The issue is that the certificate is rendered unsafe by the browser because the eb environment is showing up in the url instead of the registered domain. I get the warning as follows:

"
Websites prove their identity via certificates. Firefox does not trust this site because it uses a certificate that is not valid for <redacted>.elasticbeanstalk.com. The certificate is only valid for the following names: <redacted>, <redacted>

Error code: SSL_ERROR_BAD_CERT_DOMAIN

View Certificate"

I followed this tutorial: https://www.youtube.com/watch?v=BFh6zb43miE

Why does this not work as it should?