Hi,
I've checked the product manual. It does not mention about either VPN or IPSec pass through. IPsec protocol has two components. First of all it requires some IP packets (to be passed through) during the sending of AH phase. So it is not enough to pass UDP 500 packets.
You need a router that specifies it supports "IPSec Pass-trough" explicitly.
Given that I don't think you can use IPSec passthrough with EchoLife HG520
Cheers,
K.