@hampton123 Thanks for reaching out. From the description I understand that you have created a custom domain for the developer portal, and you were not able to access the portal using previous domain. Please note the Developer Portal endpoint does not support multiple Urls. When you assign a custom domain to the Developer Portal endpoint, the default certificate is no longer presented, and users should only use the custom domain to access the portal.
please refer: https://learn.microsoft.com/en-us/azure/api-management/configure-custom-domain?tabs=custom#considerations