This has happened before. What I was told is to lay a damp cloth on the Wonder under side and iron to get the Wonder Under hot and damp. Then try to peel it off.
Mr. Google & MSQC
Mar 22, 2014 - To remove any fusing agent that remains on the fabric, cover it with a damp, lightweight fabric scrap and press. Peel off while warm. Continue with new scraps until all the adhesive can be removed by sponging the area with rubbing alcohol--but be sure the fabric or dye will not be affected.