UTM reverse proxy authentication

Does anyone know if it is possible to configure Sophos UTM to:

1) perform certificate based authentication for reverse proxy URL

2) supply basic authentication to the backend server with anonymous or certificate authenticated  access to the virtual server?

The context:

I have a web server on the backend that performs basic auth to grant access. This server is being reverse proxied , a 3d party device need to do an HTTP post to the URL and relies on certificate auth.

I can implement certificate authentication on the backend server itself if sophos doesn't support it , however, for that to happen, an HTTP post needs to be allowed to go through. That means Sophos needs to supply a static username and password to the backend server on anonymous access to the virtual server.

In addition, for a connection to the virtual to be validated, we need to either filter by IP address of the source request or  to perform a certificate auth. I'd much rather use authentication via cert as opposed to the IP though

Does anyone have any ideas?

Thanks