FlyerTalk Evangelist
Join Date: Nov 2018
Location: Istanbul, Turkey
Programs: TK Elite Plus,BAEC GGL,ITA Executive, AFKL Gold,QR Gold,HH Diamond,Bonvoy Gold,ALL Gold
Posts: 14,185
Unfortunately as you have stepped in Frankfurt, that would count and you would be denied boarding at Kayseri or at Istanbul unless your visa type is exempted by the Presidential Proclamation.
As TK is waiving change fees, change your flight date with no fee for IST-BOS on some date later than 12 June 2021 to avoid problems.