I usually attach it to wood or rock with superglue. Rubber bands work too. If you plant it on the substrate, bury the root strings but not the woody rhizome--you want the rhizome resting on the surface. If buried it will rot.
use a string that is safe in water
if it wont get blown around by the current try placing it in a low place so it can get some nice fish poop fertiliser from substrate. my little anubias nana and his clone love it like that