Problems in dual boot with Linux Mint graphic card

[Ekki93]

I'm dual booting Windows 10 and Linux Mint on my lab computer and I recently installed (after several hardware problems) a second screen. After I solved the Windows-related issues I noticed the Linux Mint interface for selecting an user makes both screens say "wrong display mode". I could still write my password and hit enter, which made one of the screens work, while the other still doesn't. I tried updating the graphic drivers but the driver manager says I don't have an internet connection. I looked for solutions to that problem both within and outside the forums and apparently the proxy isn't properly configured (we got a proxy) for the driver manager, but none of the solutions worked yet.

Also I can't enter safe mode with Linux because I configured grub to only show windows/linux in their regular modes, and when I choose linux and hit escape I can't see anything so that hasn't worked either.

Is there anything else I can do?