Why didn't the second payment go through @Alex9620 ?
You won't get Airbnb on the phone with the host.
What did the host say when you spoke to them - are they willing to honour the booking if you pay the balance.
Unfortunately it seems you don't recollect the T&C you booked under for split payments which says the booking will be cancelled if your second payment hasn't gone through.
I have never paid by split payment so don't know if they notify if payment hasn't gone through.