I'v just installed a MX300Gen2 which will not register to UCM.
Running UCM 184.108.40.20600-5 and just installed the latest Device Pack to be sure.
There's allready a SX20 registered to the UCM. MX300 is almost exactly the same but keeps giving Failed: 485 Ambigious and I don't know why.
Though that there was perhaps a difference in device type between MX300 and MX300Gen2 but the Dev-Pack should have solved it (still seeing just one).
Difference between SX20 and MX300Gen2 is that the SX20 is running TC6.3.0 and the MX300Gen2 is running TC7.0.2