How to access the sites?

Question

linux

Good afternoon. Reinstalled OS from Ubuntu switched to Mint.
I am a teacher and I access the Internet through edu.tatar (in Tatarstan), but after I installed Mint, I stopped visiting other sites, only food.tatar and mail work, and then I can’t download material from mail.
I get: Unable to access site. Check your proxy and firewall settings.
I downloaded proxy extensions (ininja) on chromium, but it still doesn't work. please help me how to solve this problem?