Enable Jumbo Frames Causes External Hostname Problem

It presumably also fails if you use the external IP address? Also, I am thinking that the computer webserver test was also using the external host name?

Can you try ping 192.168.1.1 -f -l 8972 Also ping xxx.xxx.xxx.xxx -f -l 8972 where xxx.xxx.xxx.xxx is your current external IP address.

BTW, just to amplify something both dsm1212 and chirpa said. When you use the external IP address/host name from within your home network, the traffic still stays within your home network. It will flow upstream to your router, which should then see it's own IP address, and apply its inbound forwarding rules - which will redirect the traffic back downstream to the NAS. My router will respond to ping and tracert to the external IP address [from inside the LAN] even if it is configured to block incoming ping - since the ping is actually going outbound.

Also, though your ethernet switch will normally bypass the router if it knows the layer 2 mac address, if you use the external IP address the router cannot be bypassed. Saying that a different way - if you use the NAS IP address, the traffic never touches your router. But if you use the external IP address, it takes a different path and will go through it. I am thinking that your router doesn't support Jumbo frames, and that may be part of the explanation here.