PMM - Server cannot connect to the Client (ext IP --> int. IP)

We have installed PMM server and are trying to connect the very first client to monitor. The client machine (Ubuntu) has internal IP, and the external IP --> internal IP forwarding is being done outside of the machine itself. Hardware vendor says all NAT/port forwardings are in place accordingly, but the connection does not work : pmm-admin check-network reports the connection section Server–>Client as being down.
Have we missed anything, or is there any check-list we could submit to the vendor to verify that NAT/port forwardings have been set up correctly for this?
Any ideas or advices would be welcome…
Thanks in advance