The suction cup will not adhere well on the curved portion of the glass.

If the phone's back cover is cracked, the suction cup may not stick. Try lifting it with strong tape, or superglue the suction cup in place and allow it to cure so you can proceed.

Due to the curved glass, you will be pushing up, rather than inserting parallel to the plane of the phone.

If you have trouble, apply more heat to further soften the adhesive, and try again. The adhesive cools very fast, so you may need to heat it repeatedly.