Your problem is that you have two routers, the Sprint and your Netgear. You need to get rid of one.
You should log into the settings for the Sprint 645 modem/router and disable the router part. You do this by putting it in RFC 1483 Bridging Mode. Before you change the settings, you may want to see if there is an option to export/save the settings to the PC. That way you can always restore them later if you need to. Also note down the login settings for your ISP (not the administration username and password for the modem itself) as you will need to transfer those from the Sprint 645 modem to the Netgear router. These are specifically the username, password and usually a method such as PPPoA or PPPoE etc. Some ISP use DHCP as the method which doesn't require a username and password. A few ISPs use hard coded IP settings but you'd probably have to pay a bit more from Sprint to get those.
Now plug the internal port of the ADSL modem into the WAN port ( in olden days you use to need a special crossover cable for this but most new devices handle it for you). Turn both of the devices off, and turn the modem on first and wait for the ADSL sync to finish. Then turn on the Netgear. You should be able to log into the Netgear (put in the new settings if you still need to) and check status page on the Netgear to make sure it is connecting to the internet.