Make sure the ip of you computer is in the same sub-network with your router. Besides, reset your router via the reset button.

Routers can't be management by SNMP

check your UN&PW

I haven't configured a router's DHCP server as PXE Server before. But I have ever successfully wake up my PC which connected to the LAN side of 6120 from outside network by config a Virtual Server

remot subnet should be different from the local subnet. its the subnet at the other side of the VPN tunnel

(1) you should turn the SIP ALG on (2) that value is unchangable.