SBS2011 box behind router behind modem setup

Your help in setting up an existing sbs server  is appreciated.

The background:
We have an sbs server that used to connect directly to either our cable modem or our dsl depending on which had an outage.

Both the cable modem and dsl modem had the same IP address (192.168.0.1) with deco turned off and appropriate ports forwarded to the sbs server which has up address (192.168.0.2)

One day the cable modem hiccuped and dhcp turned itself on which caused havoc with the sbs server so now the server is only on the dsl modem.   We can't figure out where the dhcp is on the cable because all settings are the same.  

We still want to use both when there is an outage on one, like now that our dsl is down.  

How can we set up a router between the sbs server and either modem?

Do we use a router, turn off dhcp, and have the cable modem port forward to the ip address of the router?   Then have the router port forward to the sbs server?

If so what IP address do we assign to the router?  Do we turn dhcp back on on the cable modem and dsl modem and still port forward to the router?

I'm stumped and your help is appreciated.