i want to know if it’s legal/possible to connect the horizon box and at the same the antenna cable to the same tv.
Can i have problem with performance ?
The Horizon box requires a return chanel which is either found on the data plug of your wall outlet either on the adapter provided by UPC for old outlets.
There is no issue connecting your tv to the other plugs on the outlet.
May you take a photo of your setup. I don’t have any Horizon Box at home, so I don’t how the box is connected to your TV.