I’ve heard breastmilk up the nose can help break it up (never actually tried it) or a saline spray. I have been using a snot sucker to get the excess every few hours with my 4 week old and it’s been working well.
I have used the snuffle baby nasal spray and it did help my little boy. He didn't like it but it does say to use just before a feed which we did that so if he got upset we were able to sooth him with his milk x
We use saline drops/spray to break up any snot and then try to suck as much out as we can throughout the day/before bed. We also used a little drop of snuffle babe vapour bath bubbles before bed aswell I'm sure you can use it from 3months. The menthol mixed in with the hot water helps to ease her chest and nose too x
I don't know where you are but a saline mist mask (if your kiddo doesn't like it being sprayed directly into their nose) can help lubricate and break up the congestion also try to suck some of it out.