On ER5, under DHCP relay, no need to configure eth1
===EDIT====
eth1 needs to be configured, so relay deamon listen for DHCP return packets (offer and ack)
ER5 redirects DHCP requests to 192.168.123.1 .
But ER-Pro might not be listening on that interface for DHCP request. (restart its dhcp service, and look into /var/log/messages)
Worth trying:
-On ER5 use 10.92.101.1 as forwarding address
OR
-On ER-pro , add dummy scope for 192.168.123.0/24 range