Hello, thank you for your question!
For now, the best treatment will be non-pharmacological: I suggest nasal hygiene with a saline solution, such as Hydrasense, Sinus Rinse, or other brands. By cleaning the sinuses from the "front", you can reduce congestion and prevent runny throat, and you can greatly reduce coughing. This can be done as often as desired during the day, there is no maximum. It's a bit unpleasant but very effective!
That being said, the syrup can be stopped since you report that it is not effective.
A Vicks type ointment can also be applied to the chest or back, if needed.
Finally, drink plenty of fluids, cold or hot, as preferred.
I hope I have been helpful and I am available if you have any further questions!
Tom Samaha, pharmacist