I have met a similar problem as we are using a BAS that have ARCNET as the subnet for all devices and then have a BACnet/IP router to talk other IP network. In an older version of Mango (core 2.0.5 and BACnet 1.4.0), we were able to get the BACnet points by specifying the Point details as this:
IP: The IP address of the BACnet/IP router registered on the IP network.
Object instance number: The instance ID of the object (one BACnet point) that you want to communicate with.
Object type: The object type of the object (one BACnet point) that you want to communicate with.
Instance number: The instance ID of the device that directly connects to the points that you want to communicate with.
Network number: The network number of the subnet that is directly connect to the device. Typically an integer.
Network address: The network address of the device in the subnet. Typically an integer.
Giving all these information, we were able to pick up the points on the ARCNET subnet.
Recently we tried a newer version of Mango (core 2.1.2 and BACnet 2.0.3). The terminology of the point configuration has been changed. Now this way still works for us:
Network number: Same as before.
MAC: In our case, it is the Network address number.
Link service IP: Same as the IP as before. But need to add the port number now (e.g., 22.214.171.124:47808)
Device instance number: Same as the Instance number.
Object instance number, object type: Same as before.
Hope this could help you set it up. Thanks!