Bianpie Welcome to the community!
We will be happy to answer your questions as follows:
Actually I have a second wall box (3 connections) in my bedroom: can I use this second one in any way?
Yes, you can use all the cans in your apartment.
Do I need a second modem?
You don’t need a second modem, because this would be chargeable and requires another slightly cheaper offer.
I could use it to have a better wifi signal in my bedroom?
To improve your WIFI signal, you can download the Connect app from the App Store, install it, measure your WIFI and if the app tells you that the WIFI signal is weak, you can order a powerline adapter or WIFI booster.
Greetings
Daniele