Hey there Gavrilidzzze,
Thanks for visiting the Apple Support Communities to get help with your Apple Music student subscription.
According to the details in the Apple Support article you found your way to the communities from, Get an Apple Music student subscription - Apple Support, UNiDAYS is responsible for supporting your student account.
Since you have already spoken with them and they have directed you back to Apple Support, you will want to get in touch with an Apple Music subscriptions experts at Apple Support for further assistance. They will have the tools and resources needed to further investigate what is happening. You can do that here: Get Support.
Have a lovely day!