I used to use saline drops plus a little handheld nose cleaner pump. After I tried this https://www.orrszivoporszivo.hu/?menupont2=4 looks scary, but is absolutely fantastic and quick way to clean LO's nose.
Also, some steam can help and here in Belgium you can get a eucalyptus cream (like Vicks, but not Vicks) which can be used from 6 mnoths upwards.