Was this server built using the standard install guide, or some other third-party package?
Is :80 specified in your app.yml at the end of the domain name? Check DISCOURSE_HOSTNAME - it shouldn’t include the port, but likely does based on what you’re reporting above. Remove the :80 from that line if so and rebuild. The only place :80 should appear in the YML file is the expose: section.
Force_HTTPS is required when Let’s Encrypt is enabled, the mixed content error is the result of an install without it enabled.