Make the syrup: Add sugar and water to a saucepan. Bring to a boil, then lower the heat and simmer for 10 minutes. Stir in lemon juice. Let it cool completely.
Mix the dough: In a bowl, combine semolina, flour, eggs, yogurt, baking soda, lemon juice, and salt. Stir until you have a thick but pipeable batter.
Heat the oil: Pour enough vegetable oil into a deep frying pan. Heat until hot but not smoking. Test with a small drop of dough—it should bubble right away.
Pipe the rings: Spoon the dough into a piping bag fitted with a star tip. Pipe circular rings directly into the hot oil.
Fry until golden: Cook the rings, flipping once, until both sides are crisp and golden.
Dip in syrup: Remove from oil and immediately dip into the cooled syrup for a few seconds.
Serve warm: Place on a plate, let the excess syrup drip off, and enjoy.