We (SIL International) have purchased several UD-3900. They work fine in our newer buildings with Cat 6 network cabling, but to save money our older building used 2-pair (pair 1 & 2) to save money. In these older building the Monitor, mouse and keyboard works, but the network cable does not work.
The second is that if the two pair configuration was used to ‘double-up’ the amount of network drops that could be run using a single cable (one physical cable with eight conductors with two pairs terminated to two separate RJ-45 jacks) there is the potential for problems.
While the configuration you describe is not supported, based on your original description, “our older building used 2-pair (pair 1 & 2)” there are two things to double-check that could possibly help.
The first is that the network cable used meets or exceeds the CAT5 specification. Lower grade cables (such as CAT3) will not work at 100Mb.
I found one user who was using her UD-3900 Docking station Ethernet port successfully with only 2 pairs of a Cat-4 cable. This caused me to dig deeper. I found that the port on the Dell Switch had been manually set to 100 meg (Speed 100) and that power saving was turned off (No green-mode eee). So I changed the switch-port for another UD-3900 user and now she too is able to access our network. It appears that there is a work-around for older building with split cables. Thought this might help someone else.