I can't participate to the U1500 Rapid Arena

I have played more than 20 rated games in rapid and I'm under 1500 in rating but Lichess says that I ain't a <1500 player so I can't participate!
What the hell is going on with that system?

The Ux arenas go based off your weekly average rating for that time control.

And what does that mean? That I will be permitted to play the next week?

It means that you have been over 1500 at some point not long ago, and to avoid sandbaggers, there is a system that stops players from playing uXXXX tourneys if they were above that rating recently