Cisco 1720 with multiple networks

[nortelfan]

I have a cisco 1720 that is doing simple static routing to three other devices - a wan router, firewall for internet, and a ptp router. I have three networks, 10.75.18.0/23 and 10.75.168.0/24 and need them all to talk. Up until now all my devices on 10.75.18, 10.75.19, and 10.75.168 have all had their default gateway set to the wan router. I have no access to this router and would like to use the 1720 to do my simple routes on these networks (set up a small core router until we can upgrade to something more substantial). It is currently working fine, however, I noticed when I set a device to use the 1720 as a default gateway, I can no longer ping the device from the 1720.

This came from troubleshooting a second problem of someone on the outside of the wan router needing to get to a device on our network. I can ping the individuals machine fine, but they cannot do the same to mine. One way communication only. I am at somewhat of a disadvantage not being able to work on the wan router and wondered if someone could tell me what I am missing.

My router has one fastethernet port and an address on the 10.75.168.0/24 network.

Thanks