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I have a FAST5366TN-A modem, I've been trying to ping it from my public IP. I find that the ping requests get dropped and time out. I know that I need to enable ICMP requests, I'm unsure how to do this.
You can allow ICMP packets into your network by going to Network Settings-->Port Forwarding and providing the IP address of the device that will respond to it.
Because you can't specify the routers IP, I don't know if this will allow you to ping your public address though if you don't have another device to respond to it.