Whilst you await your i-pad and router*I would double check that the USB thumb drive you are using is not the cause of your problem.
Rossini should recognise the drive once the USB 2 input has been selected by pressing the button on the fascia ( that is button E in the user manual or top row of buttons second to the right from the CD drawer) and you then hold down the Menu button ( third to the right) for a couple of seconds to open the browser. I daresay that the remote also allows selection of the input ( I don’t know if the Rossini remote is the same as Vivaldi).
My understanding is that you are doing the above steps and nothing is happening. If so, have you a device other than the computer where you formatted and loaded the USB drive that can read it e.g. another computer? Some brands of drive can be unreliable which is why I think that such a check is useful to establish that it is the definitely the player and not the drive at fault.
*You say that you haven’t got a router but I am assuming that you must have in order to post your messages here or are you posting via a phone and 3 or 4G or ,say, from outside home?
You also asked earlier " Please, could you point me to a good router with wifi and USB plug for such use." The connection from the router to the Rossini is by ethernet cable e.g. Cat 6, not USB ( NB: your NAS and the Rossini will both plug into the router , the wifi connection to it will be by your i-pad) I would not recommend buying the cheapest such cable nor the most expensive but one that has been properly designed for purpose. I like the German Meicord which seems a good balance of high performance, rugged construction and price. As for the router itself I do not know how French ISPs act but here in the UK the ISP usually provides a free one with the account and they are mostly fine to start with ( and you get set up support). Then with experience you can see if you require anything more sophisticated. Streaming music , even hi-res from someone like Qobuz, does not make exceptional demands upon data capacity.
I hope I am being helpful. Let me know if you have any success.
Edit: I have just re-read the whole of the thread and have realised that the rotary control is not even adjusting the volume. That would have nothing to do with the input selected so I am sad to say that it looks like a return of the Rossini to your dealer for attention may be necessary.