Interconnecting Netgear DSL Router to Cisco 3550 Router for Internet Access

in CCNP
I've about pulled what little hair I have out of my head on this one, and need some configuration help.
I have a Cisco Catalyst 3550 switch with five Windows 7 desktops, an Avaya PBX and five Avaya IP phones attached. All of these devices are on a 192.168.0.0/24 subnet, and are communicating properly. I will refer to this as network # 1. I also have SEPARATE network, we'll call network # 2, using AT&T ADSL service and a Netgear 4-port/wireless router/ADSL modem combo device, which is functioning properly with a couple of other Windows 7 desktops over its own wired Ethernet network, using DHCP, and also on a 192.168.0.0/24 subnet. I thought it would be a simple integration, just plugging one of the 3550's ports to one of the DSL router's ports, in order to give the five Windows 7 desktop computers on network # 1 internet access via the DSL modem. Guess I was wrong. When I connect the two switches together, although I get a good connectivity (green lights on both ports) and am able to ping the DSL router's gateway address (192.168.0.252) from network # 1's computers, the computers on network # 1 cannot access the internet. Also, the working computers on network # 2 lose their internet access as long as the two switches are connected together. I am not a Cisco guru, but there's got to be a way to make this scenario work. Can someone provide me with a 3550 configuration that will allow me to extend my internet service from network # 2 on the DSL router to my 3550 switch and their computers? Here's what I am looking for:
INTERNET ---> ADSL MODEM ---> NETGEAR ROUTER ---> CISCO 3550 SWITCH ---> NETWORK DEVICES WITH INTERNET ACCESS
I have a Cisco Catalyst 3550 switch with five Windows 7 desktops, an Avaya PBX and five Avaya IP phones attached. All of these devices are on a 192.168.0.0/24 subnet, and are communicating properly. I will refer to this as network # 1. I also have SEPARATE network, we'll call network # 2, using AT&T ADSL service and a Netgear 4-port/wireless router/ADSL modem combo device, which is functioning properly with a couple of other Windows 7 desktops over its own wired Ethernet network, using DHCP, and also on a 192.168.0.0/24 subnet. I thought it would be a simple integration, just plugging one of the 3550's ports to one of the DSL router's ports, in order to give the five Windows 7 desktop computers on network # 1 internet access via the DSL modem. Guess I was wrong. When I connect the two switches together, although I get a good connectivity (green lights on both ports) and am able to ping the DSL router's gateway address (192.168.0.252) from network # 1's computers, the computers on network # 1 cannot access the internet. Also, the working computers on network # 2 lose their internet access as long as the two switches are connected together. I am not a Cisco guru, but there's got to be a way to make this scenario work. Can someone provide me with a 3550 configuration that will allow me to extend my internet service from network # 2 on the DSL router to my 3550 switch and their computers? Here's what I am looking for:
INTERNET ---> ADSL MODEM ---> NETGEAR ROUTER ---> CISCO 3550 SWITCH ---> NETWORK DEVICES WITH INTERNET ACCESS
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