Before anything else, be honest with yourself about whether your traffic quality is the real story here, because if it is, none of the below helps and you'll find out fast.
Assuming it isn't: 1) cap your spend on them today. Not as a threat, no announcement, just stop the volume. If someone comes back inside an hour asking why the traffic died, you've learned the money exists and you matter. If nobody comes back at all, you've learned something worse, and you learned it without spending another week's budget finding out. 2) Get off chat. Email the AM, cc whatever billing address is on the invoice, state the amount, the terms date and one question: what date will this be paid. Not a complaint, a date. Chat is for chatting; email is what people forward internally when they want cover.
- When they do answer, ask to move to weekly on a lower cap rather than net-30 on a big one. Most networks say yes, because it's cheaper than losing a buyer, and it caps your exposure at about a week's spend from then on. I run everything weekly now and I sleep.
Four days late is annoying and survivable. Two weeks with no written reply is when you write the thread naming them.