remote Wake On Lan from internet (WAN)

I have problem to wake up my NAS storage (readynas from Netgear) from internet. WOL works perfectly when on LAN all the time with broadcast packet on port 9.

WOL over the internet works for only an hour after the NAS (or something else) has been shutdown. I was read lot of forums and no configuration helps (set port mapping to broadcast address .255, different port 7,8850, etc). I thing the problem is in ARP timeout. After mac address from NAS was delete from ARP table, airport drop every WOL packet and don't send to NAS.

How to fix ?

1) ways to set static ARP

2) ways to change ARP cache timeout

3) proprietal hack when WOL packet comes to AirPort make broadcast to LAN

4) another solution :-)

Thanks for help

MB