You need to replace the router or the modem ? Your title says modem but then in the post you say router but list a phone modem as what you have.
Can you straighten this out so we know what your lookiing for?
If your wanting to replace the phone modem , usually the cable company supplies that piece of equipment and you can buy it yourself but they are hard to find.
If you go back to where you bought it from originaly then you can get another. Surfboard modems are very good and reliable.
I used to work as a sub-contractor for a cable company and I was always happy to see a supply of surfboards come in as they worked the best of all the modems hat were used.