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1)In Your client machine(LAN)you should have SSH installed
2)In router 'Add service' under Advanced > Port Forwarding
3)Select Common Service as SSH service and Service Type as TCP, then enter Server IPv4 Address where ssh server is installed, port is 22 and save.
4)Try ssh for lan client device using gateway ip address, we should able to access lan client through gateway ip using ssh service.

UDP - The Netcat (nc) command is a command-line utility for reading and writing data between two computer networks. The communication happens using either TCP or UDP

1)In Advance > Port Forwarding > Add Service, Common Service as other, Service Name as Netcat and Service Type as UDP, add lan clients's Server IPv4 Address 
2)In Your client machine(LAN)you have to run : nc -u -l 2399
3)From external WAN machine run nc -u 192.168.2.30 2399 (192.168.2.30 - router uplink ip address) 
4)In your client machine(LAN)run https service(In browser https://www.google.com)
5)Goto to your external network(WAN), access server of client machine(Try sending some packets to wan client). Able to see incoming traffic on ports 2399 after seeing an outgoing packet on port 443