What the checkbox in the GUI does, it turns on the Jumbo frames support in the switch. I'm not even sure if that works properly, and whether it affects LAN ports only or all WAN and LAN ports since there's no docs for the switch. Simply activating it on the switch is probably not enough to have it "working". You may try and change the MTU on the interfaces as well.
Also, the "extras" package contains the robocfg utility - it too can change the Jumbo Frames flag in the switch, but unlike the GUI, it can do it on the fly without rebooting (but reinitialization of affected interfaces may be needed after that).
If you play more with it and figure something out, please post your findings here.