Originally Posted by
airoli
I don't want to presume for the OP, but it is quite possible that IST-FRA was on TK, and therefore TK agents in IST denied OP boarding...
TK would sell one of their direct flights to the US, with a final domestic leg on UA if needed.
AFAIK TK don't codeshare on other non-Turkey TATL routes (happy to be corrected if that isn't true) and a TK-->LH connection on a single ticket is not going to be cheap (again I don't think LH codeshares with TK). Perhaps using miles or separate tickets, but I would still wager that a newbie OP jumped to Turkey rather than the actual airline. Won't know until the OP updates us...