Inserters with matching power supplies from other manufactures of unicable II LNBs suggest a 19V supply voltage.
Into which downlead are you injecting the power? It should be the unicable output and not the legacy.
When injecting power via an inserter is there a facility to stop the box also providing power to the LNB - the two power sources could be fighting each other?
How much current can your external power module supply - the spec suggests 300mA for the unicable side of the LNB ?