Try really hard to find the model number of the 3com. I'm sure it's on the box somewhere.
Set the IP address of the 3com something router to an unused address in the same Class C IP block as the Cisco 800 router (without duplicating any IP's). Do NOT use any IP addresses that are inside the DHCP servers IP address pool. Do the config from a directly connected PC.
Make sure that the DHCP server in the 3com whatever is disabled.
If you're running a DHCP client on the unspecified model 3com OfficeConnect, verify that it is getting and IP address by looking at the DHCP address table (or ARP table) in the Cisco 800 router.
Since you're apparently using a 3com something router as an access point, do not plug anything into the WAN (internet) port. Connect the CAT5 cable that eventually goes to the Cisco 800 to one of the LAN ports on the 3com something. Check the lights to make sure the ethernet cable is working. If no lights, then you'll need an ethernet crossover cable or adapter.