|It would appear you are connnecting the downstream router via its wan port.|
If that is the case you can't disable dhcp on it unless you do a static assignment to the pc.
The pc and gateway entry can not be in the same subnet as your existing lan. So if the gateway to the internet is 192.168.1.1 the downstream router needs to be 192.168.0.1
Normally you would just turn the ds router into a glorified access point by connecting via only its lan ports and configuring a static ip in your first routers subnet.