So I’m thinking, how can I get backend from RF if the tracks aren’t registered on PRS and are truly royalty free ?
Royalty free is a misnomer. What a customer, on a RF site, is doing is buying a license. On some sites the license is for only one project. On other sites it could be multiple projects, in perpetuity.
There are royalty payments, depending on the RF site’s policy towards PROs. I have received backend royalties for tracks licensed from an RF site, used for commercials.