E6000 glue or shoe goo adhesive. This was previously either heat bonded or ultrasonic bonded - doubt any has one of these in their garage. Your next bet is the old fashioned way - just resewing it on with an adhesive backing. Don't do one without the other - as they'll work together for a much longer lasting repair. Depending on how you carry it may dictate the type/thickness of thread that you use. If the plastic on the strap is too hard to poke through with needle you'll need to drill or pre-poke holes (with an ice pick or thumb tack or similar).