Fasting seems to help with a number of things - it can’t hurt to try. I’d also recommend Navage - like an electric neti pot! Some drs will suggest adding a drop of povidone iodine to the saline solution.
I’m experiencing this right now (day 6 of a 30 day fast.) I usually have REALLY bad and aggressive sinusitis that requires I take allergy medication during the day and Benadryl every night. I haven’t taken either since day 3.