I'm into consensual BDSM and I need a sub

Ok thank you but I'm not giving up on TER

The subs are definitely there on Fet, but many (most?, almost all?) of them are very relationship-oriented.  Open trolling for pickup sex with subs will probably be frowned on a bit, that community is protective of them.  If you want to find a sub in that community, you might try visiting some of the local munches (public) and get to know some of the community members first.  That could lead to a visit to a local venue or dungeon for a social, where you'll meet more people.