The volume on my TV increases by itself.

Hello Cricri, and thank you for the super quick response!
Actually, I completely removed the batteries from the television remote (it's a bit broken...), I'm using the TNT decoder remote, but when the volume goes up, I have to adjust the sound directly on the TV buttons..
And despite that, the sound keeps increasing....

Hello, two possibilities: 1/ The volume buttons +/- located on the TV are worn and the volume + button is sticking or getting stuck
2/ The remote control of your TNT decoder affects your TV; this can happen, even if the two are of different brands
For example: I have an old Strong TNT decoder, when I press 'p+', it turns on my Tokai DVD player!
Similarly, on another low-end Castorama TNT decoder: the menu button turns on the Tokai DVD player (both Tokai DVD players are recent)....
Test each button on the TNT remote control to see which one is guilty of the issue
Tip: Manually set your TV volume to halfway, then don't touch it anymore and cut a piece of opaque plastic, for example, a piece from a DVD case, and place it in front of the TV's infrared receiver, using repositionable adhesive (like Patafix); this will block any potential signals from the TNT decoder interfering with the TV, then adjust the volume using the TNT decoder remote only
This tip only works if the TNT decoder is the cause; if the TV button is faulty, the tip will be ineffective
Conduct the tests....