Secure Connection Failed: the authenticity of the received data could not be verified

I created a WordPress instance using lightsail and assigned a domain to its static IP and enabled HTTPS on the WordPress instance too. The web app is accessible everywhere, but when the user is behind proxy/firewall, he gets the following message and the browser does not allow access to it:

Secure Connection Failed: the authenticity of the received data could not be verified.

How to solve this issue?