If you have never done a SIP trunking implementation before, you are in for a long hard road.
-For starters, put the IPO directly on the internet so that you don't have to worry about NAT transit issues. You can try to put it behind a firewall or NAT later, but believe me you don't want to have to worry about that yet. You will have plenty of other issues to deal with.
-Make sure that you know how ARS works. If you don't have your short code and ARS configured correctly, it will never work. You will probably need to append the outbound dial string with the IP or URL of the SIP ITSP.
-You must have the SIP gateway on LAN2. It won't work on LAN1.
-Once you have LAN2 configured correctly, you MUST run the STUN server search and it MUST respond that it detects open internet. If it doesn't, then it will never work.
-You will need to setup your SIP trunks correctly. Unfortunately, there is very little accurate information out there regarding configuring this section correctly.
-There is much more that needs to addressed, but it would take too much time to do so. Read all the documentation that Avaya provides and then start testing.
This should give you a starting point.