Is it a Saas? Either way, Call them. Yes call. Ask for accounts payable. Ask what the problem is, missing invoice, missing approval etc. Ask them how to fix this. Ask them for a payment date. This is what credit controllers do all day. Don't threaten, just ask.

If they are evasive, then state when you will be withdrawing the service.

Next month before it is due, call them, check they have the invoice on their ledger and ask the payment date. The squeaky hinge gets the oil. Eventually it gets easier to pay you than deal with your calls. Accounts payable people often have more discretion than they will ever let on about who gets paid when. They put the people who call higher up the list to save the aggravation

Escalations. Tell them that if you continue to receive late payment then you will have to withdraw credit and put them on advance payment.

Tell them you will have to 'take it further, which may include collection activities' or 'recoveryaction'. Deliberately broad.

Hold future work.

Write them a letter stating their account is in hold. Enclose a copy of the statement of account and invoices.

Next you will need to find a debt collection lawyer in your country. In the UK you fill out an online form and pay about £30 for a Letter before Action'... actually called something else now. Let the lawyer guide you from there.

Always send statements, always call.

That is how credit control is handled professionally. It is part of my job to manage this at several companies.