Could be Viral Upper respiratory tract infection with initial prodromal symptoms without fever or cough, other diagnosis could be nasolacrimal duct blockage but yellow mucus in nose is not explained by this and could be allergic or non allergic rhinitis as well but none will explain a yellow mucus in nose as it is mostly due to bacterial infection if it's yellow thick.
Next Steps
Consult a nearest Pediatrician
saline drops 2-4 hrly
antiallergic drops
Health Tips
danger signs fever, cough, symptoms getting worse, decreased intake of feeds to review by nearest Pediatrician. Also if symptoms do not improve in 3-4 days without getting worse then needs to be consulted.