[OnePlus 7 Pro Questions & Answers] djsubterrain:
If the volume is stuck on 0, check the notification slider on the right;

Top is mute all except alarms or "Silent" (Great for setting an alarm to wake yourself up without being woken up by stupid alerts and app noises)

Middle is Ringer off but media sounds enabled or "Vibrate"

Bottom is all sounds on and vibrations on or "Ring"

Although you might need to use the volume buttons to turn the volumes back up if they had been adjusted while the slider was moved, just move the slider to the bottom position then use the volume buttons to turn the volume up. You can also check Settings>Sound and vibration to see what the levels are at and use the sliders there too.

If you're a noobie then DO NOT TRY TO ROLL BACK THE PHONE, if you don't know what you're doing then you're highly likely to get into problems. There's nothing wrong with Android 10 on the OP7Pro, believe me.

Also, You don't need to "install Android 10", you're already on it.

If you've bought a resealed phone then I would recommend wiping the data and treating it as a brand new phone. Reboot the phone and hold down the volume down button which should take you into the recovery menu, go into recovery, then data and wipe data and cache. Then, when you reboot the phone it will be ready to setup as a new phone.

To ensure you've not received a tampered phone, when the phone is booting check for a screen saying "The boot loader is unlocked and software integrity cannot be guaranteed", if that does not show up then the OS should not have been tampered with and the phone should be safe. If it does then that doesn't necessarily mean it's dangerous, as the previous owner may have just installed a custom recovery (TWRP) and rooted the phone or tried some custom roms, but you should think about re-installing the OS if the bootloader is unlocked, download the OS from here :

https://www.oneplus.com/uk/support/softwareupgrade/details?code=PM1574156267635

OnePlus's official forums are here, you should think about registering and keeping an eye on them as you'll see announcements about new versions of the OS as well as beta versions, although if you're not confident flashing roms then you might want to stick with the default one that OnePlus pushes to your phone.

Also EMAG seems to be a Romanian store? So, if you go into Settings>About phone (at the bottom), then tap on Android Version, your installed OS version should be shown under "Build number" at the bottom of that page. EU versions should show "BA" as the last 2 characters, Global versions show "AA". The latest for each at the moment is :

Oxygen OS 10.3.0.GM21AA (Global)

Oxygen OS 10.0.3.GM21BA (EU)

Although you did say your build is already BA so it should be correct if you are from Romania? If your data isn't working then I would be checking that the phone wasn't locked to one particular supplier in that country but you may get away from flashing the European build from OnePlus's downloads above.

Don't ask us why they're using a different build number cos we don't know either, possibly to avoid confusion for users? In any case, you shouldn't have any problems with data connection but, if you do and it shows "AA" at the end, then you could try flashing the EU build instead. Just download it from the link I provided above, place it in the root folder of your phone's storage and then go into Settings>System Updates (Bottom), then tap the cog on the top right and go to Local upgrade, you should see the rom you downloaded there. Select it and follow the instructions. To save you getting into trouble, I would recommend removing your pin or screen lock before doing so.