In agreement with Ken, two things fall down here...
1. The 1761-NET-ENI routing table just maps the DF1 node address to Ethernet IP addresses. It cannot map the SLC's source link ID, which is required for the 1756-DHRIO, so all it passes to the 1756-EN2T is Ethernet data.
2. Because the 1756-EN2T only receives Ethernet data and no source link ID, it would need to map the Ethernet data to a source link ID before routing it to the 1756-DHRIO. But the 1756-EN2T does not have a routing table to do this, so the data stops at the 1756-EN2T.
If you had a ControlLogix processor in the chassis with the 1756-EN2T and 1756-DHRIO, and it was in slot 0, then you could message from the SLC 5/04 to the ControlLogix processor first and then relay message that data out the 1756-DHRIO, but it doesn't sound like you have a processor in the 1756 Chassis?
Regards,
George