FreePBX behind a Nat in Greenpacket Dx-250 router, client with public ip can't register

I have a FreePBX which is found behind a Nat in a Dx-250 router with a fixed public ip and the sip clients have fixed public Ip’s, they have no natting since i am using the voip service that is built in the router"green packet Dx-250 supports two lines".
I forwarded the required ports for the pbx at the router and added its public ip to the externip in Asterisk settings also i added its local ip and clicked yes for natting.
The sip client are not registering at all. I presume there is no natting on the client side since they are using the dx-250 voip service.
**In configuring the sip clients i am using the sip public ip.
**The PBX is also behind the green packet dx-250.