What kind of adhesive should I use to glue the case back on? Thanks!

I guess I may have not soldered the sleep/wake sensor right and it was not working after I put the new screen back on. I wish there could be a way to test if the soldering was good or not before I connect the screen back. Also, I have found that even the sleep/wake button near the upper right corner even cannot put the ipad into sleep or wake it up by pressing it once. I guess this could related with the sensor, and the lock/unlock option under the "display & brightness" setting has also been gone due to this issue. So I would say this sensor is not only for the smart case, but also for the sleep/wake button to work as well (the other functions of this button such as poweroff, taking screenshots are still working fine). Solder it carefully if you still want the button to work.