Cisco 881 router

I need your help guys i am stuck. I am new with cisco routers.I have a cisco 881 router, i have configured my FE4 with DHCP address because i have a vsat modem which supplies IP through DHCp.
I have configured my VLAN ip address.
My problem is that i cannot connect to the internet through the modem.
My problem is that i cannot enable frame-relay ietf.
Help