Ok, thank you for the helpful responses so far.
HLDS does accept connections from many IP addresses within the local LAN however it doesn't respond to more than one WAN external IP for some unknown reason.
I've successfully made it work with both WANs and my players are very happy, they can now choose which connection is less laggy for them and that's great for me since I am balancing the load between them.
pfSense did the trick, a FreeBSD based firewall I set up on a mini-itx box with a dual lan intel motherboard plus a dual wan pci-e low profile card. Works wonderfully, either IPs accept connections and I've been using it for a couple of days.
Was pretty easy, except for finding the low profile gbit card.
Thank you