Hi CG,
Not sure, if I understand your issue completely, but as you mentioned that backup RE is not able to reach any other subnets except the directly connected subnet, it would be good to check "show route forwarding-table destination <>" for the route you are trying to reach on backup RE. If that does not show any entry and you are not running nonstop-routing, then probably adding backup-router configuration might help. You can refer to techpubs link for more details:
Hope this helps.
Thanks