kjassal,
I see that there is a NEW firmware (2.1.5.4) released and is on MoL. The release notes are something of a mystery. Looks like all they did was take the 2.1.5.3 release notes and replace the cover and table of contents page. Other than that it looks all the same. I'm going to be opening up a ticket with Mitel and asking for the release notes. I'm certain that upgrading to the new release will not fix your issue, as lundah has said, getting these UC360's setup is pretty easy in just making sure your SIP settings on the UC360 match up with what's on your server.
Thanks,
-Iron