ah OK. I was just confused when you stated flights OSL to SIN and FRA to SIN, but I understand now that you mean OSL-IST-SIN and FRA-IST-SIN on TK.
Did you try to talk to your local TK ticket office? Often they are much better /more knowledgeable to get things done.
You had a cancellation or schedule change of 15min or more, and now you request an (invol) rebooking of origin to FRA free of chargfe?