But a problem with new Modem
You can stick with your current modem and buy only switch with wifi facility. But, buying a new modem will cut your mothly charges with no service from your ISP about modem.
SO, you can buy modem if you are sure that you donot need ISP to rectify any sort of issues related to modem. They may also say all the problems are arising because of your modem and they may not provide even a single service other than connection.
Think about all this and then buy swith or modem.