how to install wan ip in cisco 1721 router

i have cisco 1721 router in my office. 2 days back we are facing the problem of internet slowdown, then we call our intnernet service provider to check with and he told me that internet from their end is working fine, they suggest us to check with cicso router which internet static ip is configured in cisco router, the adsl router of internet was configure in bridge mode. we have check the adsl router by directly connecting to server and found working fine, but when the same adsl router we install in cisco router the problem of internet slowdown occur. need a suggestion