So in the ATT MPLS setup I assume this is a L3 VPN VRF for your traffic.
If that is the case you should be able to inject the default route from your data center SRX to the ATT peer at the data center. This will then become available at the remote site.
I further assume you added the static default route to the remote peer just to get the internet flowing into the MPLS connection. But since there was no default route learned from the data center this traffic basically blackholes at the remote MPLS router as a result.
Once you advertise the default route you should be able to remove the static one.