Re: MPLS Routing Fortinet with AT&T
If the public subnet is supposed to be configured "inside" interface, you must have ordered to get it from AT&T, then AT&T route that particular subnet from/to the internet to/from your MPLS circuit. If you didn't order it but they say they would provide a public subnet like a /30, that must be for "wan-side" interface to connect to thier MPLS router.
Either case, call their support. They would tell exactly what you need to do.