It looks like it is configured in access mode:
Configure the interface to be a trunk:
conf t
int gig1/0
switchport mode trunk
switchport trunk allowed vlans <internal VLAN, Guest VLAN>
trusted
trusted vlan <internal VLAN, Guest VLAN>
!
Define the VLAN:
conf t
vlan <guest VLAN>
interface vlan <Guest VLAN>
!
ip address x.x.x.x x.x.x.x
ip helper address <DHCP Server Address>