It may not be a cold or illness. My daughter did this at that age and it was milk getting in her nose. I had to start feeding her sitting up and keeping her that way 30 minutes after each feeding. I tried cleaning her nose and everything. It wasn't even congestion with a cold. She would sometimes sneeze milk. Lol

Humidifier, zarbees chest rub

I actually heard a dad left his kid in the bathroom for a little while alone to decongest him and the poor baby ended up dying , but that’s obviously bc the dad left him unsupervised but I’d be careful taking the baby in the bathroom . If just Try a simple humidifier

Steamy shower, sit in the bathroom for a bit