Cupped fingejointed hardwood Birch panel

I’m in the process of building myself a workbench. Up until now, I’ve been using the floor as my workbench.

I ordered an untreated, but planed fingerjointed hardwoord Beech counter to serve as my countertop (2m x 1m ~ 6ft by 3ft).
When it arrived it was quite cupped, both in width as well as in length.

When I asked the supplier about this, they told me to leave it for 24h on either side and it should straighten itself. I tried that, but it’s still very much cupped in either direction.

I could attach it to the bottom of the workbench and leave some space in the bolts to allow for movement, but I’m not sure I want to be dealing with this as my workbench top.

What are some other things I could do to improve this board?

Needless to say, I’m a beginning woodworker and don’t have a planer nor a jointer, nor will I get one anytime soon (and especially not one that allows for this size)