Hi and welcome
Please continue with the nebulisation (if that helps without the antibiotics and just a mild disinfectant, like saline solution, which you can make at home) or consider a humidifier in the piggy room for the long term.
See your vet again and see whether your guinea pig may need a different antibiotic (we have started seeing more cases of resistancy in respiratory bacteria due to widespread overuse of antibiotics in recent years) or whether you are dealing with a sensitivity to the environment; this can often include hay dust but also air fresheners and other strongly perfumed/scented products, very dry air from air conditioning or heating units or smoke. True allergies are very rare in guinea pigs but some sensitivities, like the one to their main food source, hay dust, are not uncommon.
Irritants to Avoid Around Guinea Pigs
It always gets difficult once the important first approach, treating for a potentially killing or long term harming URI, is not working because all other possible avenues can only be worked out by trial and error.
PS: To make saline solution at home: Boil 250 ml or 1 pint of water, let cool and stir in 1 teaspoon of water. It is a mild antiseptic that can be used for inhalation, wound cleaning/disinfection, abscess flushing etc. and comes in very handy in any emergency. You can also buy sterile saline in any pharmacy or use other comparable products recommended by your vet.
