It’ll hang on installation if (a) the box is screwed, (b) it can’t get online due to faulty wiring/cabling or © UPC haven’t provisioned it properly.
The only way forward would be for you to ring up. They don’t offer an English language option any more but they seem capable of dealing with awful German (in my case!) or you can ask for an English speaking person and they’ll put you on hold and transfer you to someone.
They can monitor the box’s connection status live while you turn it on (have it unplugged when you call) and it boots up. They’ll either fix their issue (if c), get an engineer out to you (if b) or replace the box (if a).