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Why cannot I ping into the WAN of my C1200 V2 : i.e, not responding to ping on the WAN

2.0.2 Build 20180118 rel.38979

This router is set up as a secondary NAT router, to communicate to other PCs in my house.
It is set in Router Mode.
The WAN is set to 192.168.4.99, static,
The LAN behind is network of 192.168.6.0/24, and the router's address is 192.168.6.1
There is a SSH server on this LAN with IP 192.168.6.105
The router has port forwarding set of port 22 to 192.168.6.105 (NAT Forwarding/Virtual Server menu)

From a PC on the 192.168.4.0/24 network, I
a. Can successfully ping 192.168.4.99
b. Cannot ping 192.168.6.1
c. Cannot ping 192.168.6.105 (where I know the server is set to respond to ICMP echo request)
d. Can ping 192.168.6.1 and 192.168.6.105 from the Diagnostics menu.
e. Can SSH into 192.168.6.105 from a PC on the 192.168.4.0/24 network, so I know the router is working.
f. Can of course ping out from the 192.168.6.0/24 network.

There is an option "Ignore PING packet from WAN port", in the Security menu, which is deselected.
Which gives me the thought that it should respond to PING

Somehow, this router is blocking ICMP packets.

Can anyone advise how to enable.

Thanks
John