None.
Replace the arm, or shell if that is applicable.
If DJI still ' repair' mavic airs I would suggest you send the drone in to DJI and get an estimate.
Given its age they might have to repair it but you might get lucky and they might have stock of either new drones or refurbished drones in which case they are likely to send out a replacement drone.
If you glue it back together it will fly funny until the adhesive gives and then the drone will crash. It won't take long. Putting on a new arm is much less of a hassle then trying to glue it back together anyway!