Screenshots would help, but a couple questions:
How do you have the external and internal addresses typed out in the provider?
Do you have an application attached to these providers?
Do you have them enabled in your default outpost?
Is your outpost setup correctly to resolve to a domain name?
Are you using the self signed certificate provided by authentik, or are you using an imported one?
Do you have the certificate set under Brands, and also under the providers, if using a custom cert?
Feel free to look over my Public Gitlab for help since I only use Authentik as my proxy and everything is setup using transparent proxy. I do have a cloudflare certificate imported but you can try to substitute it for the default self signed one from authentik for testing, or maybe you can leave it blank, not sure.