Is your NAT-Type 'strict', 'moderate' or 'open'?
Which router have you got?
There are different solutions:
-Activate the UpnP of your router.
-Activate the port-forwarding of your router.
* UDP 28960 -> in the majority of cases, forward this port is enough.
* UDP 1500
* UDP 3005
* UDP 3101
-If possible, activate the DMZ-feature of your router.
-Firewall -> CoD Mw2 blocked?