I started my Mini after 2 months. On thet time it works perfect, but now it has a solid red light and shows RC not connected to mobile device. Any ideas what happened here?

Read the manual for the procedure to re-pair the Mini to the remote.  It's easy.
Why it lost the connection, I can not say.

tried all versions of re-connecting but still doesnt work and still solid red light. I downgrade the firmware and of course unsaccessfully and now I even cant upgrade it. nothing zero!

Hello there Thremal drone. I am sorry for the trouble this has caused and thank you for reaching out. May we know what is the model of the mobile device you are using please and have you tried to a different cable if you have an extra to connect the RC and the mobile device. If the issue persist, kindly please contact our DJI Support Team at https://www.dji.com/support for us to be able to assist you further with regards to this matter. Thank you.

Did you charge everything. I have read where an error will occur if not charged completely after setting for a period.

My phone won't link with controller after update. All batteries are fully charged. I can fly drone by starting it with both sticks pushed inward. Tried relinking several times by holding start button for 4 seconds on drone and controller. Just won't link up after update.