Using the bsc instead of erc on metamask while buying

Hi I mistakinly payed bnb to buy a token on bsc on metamask.However I should have used ethereunm chain (erc.) And now my bnbs seems lost and I could not buy the token I payed for.
How can I save my bnbs back that I have payed for the token.Please , I 'm witing for your support.