@Aiello If I understand correctly @Lyzards looks for information about making two Waveshare DSI displays work. Assuming @Lyzards refers to this and this display the settings in post #2 won’t help.
@Lyzards:
For the 7inch DSI LCD (C) you find setup instructions here. Be aware that the instructions in the link are for a Pi 4. In case you want to install on Pi 3 the URLs in the curl commands have to be adapted.
If you have already tried to install drivers etc. I recommend to start from scratch i.e.with a freshly installed Volumio system.
Regarding the 7.9inch DSI LCD as far as I can see there is currently no driver fitting the kernel version (5.10.92) Volumio 3.324 uses available from Waveshare.