Here is my version: R80.40, OS build 294, take 78 on both cluster members.
I do not have exact information about the other end of VPN, it is a Sophos Cyberoam device, I will try to get more detail information since it is a device from other organization.
Link Selection is left as default, I am sending screenshot with my settings. I have declared other end VPN gateway as interoperable device and it has the same Link selection settings.
I have three interfaces which are declared as external in my Network topology, two of them have public addresses and this third interface has private address and it is used for connecting to other organization over the MPLS VPN service. So, we are trying to establish S2S VPN over this third external interface. Besides these external interfaces, I have several interfaces declared as internal.