I have done many repairs over the years and here is the best and easiest I have found-

cut antler straight as possible just below break.

Find a piece of antler about the same length and width as the piece that broke off. (I save all my antlers from bucks that I don't mount)

Drill a hole in Antler and in piece to install the size of a 16 penney nail. (as close to center as possible)

Take a piece of a 16 penney nail and slide 1/2" inside each. check for fit.

epoxy together.

you can make any final matches (roughness/grooves) with hot glue.

No one will ever know!!!!!!!!!!!!!!!