What to do to monitor a Customer Network

We have installed OpManager on our server (SBS 2003 Premium, 192.168.0.50), enabled snmp on our devices and it's working fine. We use a cisco 826 router (192.168.0.6) and our external address is 203.23.216.65. Next step we would like to take is to monitor a customer. This customer has an ADSL connection using a Cisco 837 Router with a static IP Address is of 203.23.221.59. We have enabled snmp on this router and detected the router by discovering the 203.23.221.0 network but can not determine (no blue star) if snmp is working neither are we able to discover any of the customers devices. Since we are new to opmanager and our knowledge is limited what do we have to do to make this work. Do we have to use RIP, OSPF on the router and if so how do we configure?
Thanks in advance :lol: