BACnet/IP breaks because the source address is changed as it goes through the NAT-Router. If there's only one device behind the NAT then you can set up port forwarding, but that's not a great solution. You can use a set of Annex-H half routers, which essentially tunnels the traffic over IP. I'd be happy to help you with that, it would make a good use-case for the BFR documentation.
BTW...
