not sure that’s it. i created an account on another instance and i do see my post there.

The backend and the frontend can be separate things in Lemmy. In this case, you go to mlmym.org and the first thing you have to do is input your lemmy server name. Then, boom, you’re browsing the Lemmy instance you entered, but it looks like Old Reddit. It’s a thing of beauty. (I’m using it right now to respond to your comment!)