Hello grayalexanderj,
The SG200 switches are Layer 2 devices, so you will still need the ISP modem/router to stay as it is now. The SG200 will just give you more network ports, to connect more devices with a cable.
You may continue using the Wireless, provided by the modem/router. On the other hand - if you need more/better coverage - it is not a problem to connect additional access point to the switch.
If with default settings - the SG200 switch is retrieve an IP address by the modem/router and provide L2 connectivity to the other devices you will connect to the network. But, it does not have any L3 capabilities, so the router will be needed as it is now.
Regards,
Rositsa Sareva
Cisco SMB engineer