I’m not so new in Mikrotik world, and I know some about networking.
I have situation, My client wants 2 computers in my office to connect to their office network via internet. They gave us a Juniper Netscreen 5GT. We just need to connect 2 computer to trusted ports and internet to untrusted one.
In my office, we share the internet access via Mikrotik, with DHCP and NAT. But when we attach the Juniper untrusted port to mikrotik port, we hope it just works, but it doesn’t. We don’t have access to the Juniper at all. We can ask question, but unfortunately, I don’t know what to ask. 
What should we do?