Hi and thanks for the query.
Most of the times this is due to acid reflux as opposed to any upper respiratory tract infection.
If you have morning sore throat, irritation of the throat, pain behind the sternal area, post nasal drainage then these are also seen in acid reflux and that is why the mucus refractory to home remedies.
The best approach to this is to visit your GP and get a referral to the ENT first and the Gastroenterologist later on so that a complete work up can be done and the treatment can commence according to the underlying problem diagnosed.
However, in the interim you should sleep with a head up tilt of about 30 degrees at night, use a wedge pillow for this purpose.
You should avoid spicy, salted, smoked and barbecued foods especially for supper. You should avoid drinking hot beverages like tea and coffee and also fizzy drinks and alcohol specially before bed time.
Add a proton pump inhibitor OTC Omeprazole 10 mg twice a day taken 15 minutes before you have your meal.
These will help along with steam inhalations four times a day with menthol crystals.