Enable Jumbo Frames Causes External Hostname Problem

Pinging the internal IP of the router and the external IP of the router both fail with -f -l 8972. I'm not really surprised by this at all. If I ping my external address from within the LAN it will respond to the ICMP, even though I've asked it not to. The ping fails when outside of the LAN, as expected.

I'm not convinced that the router is the problem and I'll try and explain this again :) If I run a webserver on a spare PC with Jumbo Frames enabled, on the same switch, same router (and configure the port forward to the spare computer instead of the NAS), then I can use the external address on the LAN with jumbo frame enabled clients. The same configuration fails on jumbo frame enabled clients when I host the webserver with the NAS with jumbo frames enabled. However(!), when I am outside of the LAN, at the office for example, then the external address will work with jumbo frames enabled on the NAS, but when I get home then it fails on the LAN.