How to set mtu at boot up to get jumbo frames on a NAS 542

Hi !
I have been looking around for some info where i can edit a file so that i can make the mtu setting get set before network card starts up.
In others nas i have hade there where an option in the dhcp settings page.
So i was hopeing to get some answers from here on how to enable jumbo frames directly at boot.
Because the only way now is to close down the card and then set the mtu and then restart the card.
But when this is done i get alot of problems after a while its not possible to login to the unit. And other stuff.
So if some one could point me to the file i need to edit for it to be able to fix it !

Many Thanks