Random mac address

Hi,
I have a radxa taco and a CM4.
I have installed pi os lite with an io board and, when i connect this io board to my network, it shows up with the same mac address each time i boot it. However, when i connect the cm4 to the taco, the mac address changes after every boot, and i can’t setup a fixed ip address with my router.
I haven’t found anything on this matter.
how can i make the mac address static ?
thanks