No issues with port forwarding here - though i don't use an ASUS device..
idk about ASUS, but do you need a corresponding firewall rule to allow the traffic through (in addition to the port forward) or does it take care of that for you automatically when you create the portforward??
it's also odd that your ASUS is reporting a 192.168.100.x/24 address - whilst there is some trickery going on with that range when in modem mode, the WAN IP of your router should be reporting the public IP address, not the 192 address..