My account got unbanned on 21st december and I am still unable to get keys and chests

There isn't a specific time frame. When the system determines that you aren't going to be toxic anymore, you will get rewards back. It's based on games you go without getting valid reports.

It takes up to 3 months after the end of the punishment if I remember correctly.

You have to not get reported, mute people and keep your cool. After a certain number of games you will get rewards back.