Cannot access OWA/ECP except on the server

Out of the blue, we cannot seem  to be able to access ecp by mail.domain.com format.

On the server we can access OWA via https://mail.contoso.com

Internally, accessing OWA works via https://mail.contoso.com

Going to our static ip address externally via https://<office-ip>/owa does not work. mail.contoso.com doesn't either.

I have restarted the server with no luck. It appears all services are running, however I had to start Microsoft Exchange Transport Delivery.

We can receive emails and check them via the server itself.

Any ideas what the problem is?

The port is forwarded to the Exchange server and appears to be open. I'm going to contact our ISP but since it's a business account, it should just work. Emails are being received on port 25.

Thanks