I would:
- buy a router (as well), the crs112-8p-4s-in is a switch (though being able to act as a router, it lacks performance)
- read about VLAN: http://forum.mikrotik.com/t/using-routeros-to-vlan-your-network/126489/1
- share your current config to get some proper advice /export hide-sensitive file=anynameyoulike