I had to set up my sister's 521, and later 925 with T-Mo-based Straight Talk and the APN settings were a pain, it took a solid 10-20 minutes of trial and error, switching around settings and stuff.
The settings had to be different on the 925 too, so I wasn't too happy with that.
Might want to poke through here, try these various settings, see what works for you.