I think you already spoke with our Support team about this separately. The NID is a passthrough device with GDTs for lightning protection, and is definitely meant to be installed between the PoE and radio: http://client.help.mimosa.co/client-installation-guide-nid-details
Since the B5 and provided PoE already have GDT's, it may be that the introduction of yet another set of GDTs has increased capacitance to a point where Ethernet will not negotiate.
There are a couple of tests that can help get to the root cause.
Perform a cable test through both sets of cables and the NID - testing them all as a system to look for wiring errors or another hardware problem.
Substitute another PoE that does not contain GDTs to reduce the capacitance.
Please let us know what you find.