DSL Modem Address

I have purchased several speedstream dsl modems models 5200 and 5360 over the past year. I have sold them to customers in our area with not much hassle. However lately I have recieved 4 modems I cannot get to sync with the dsl line. I was wondering if there was a way to find out the modems ip address and like a router tell it to release. Or if you have any other suggestions. I have run ipconfig until I am blue in the face with no help. I would like to have some control inside the modem box.