Julius It's also linked in the official topics that you probably have already seen, but first step could be to report your case here: https://www.acceptmyiban.org That's an initiative originally started by Wise that bunq has also joined in. Wise has also some interesting information about IBAN discrimination written on their blog.
The next step would be, as you already mentioned, to report it to your country's national authority. Refer to this list: https://ec.europa.eu/info/files/sepa-compliance-competent-authorities_en Your country might have two different authorities or just one. If it has two, think about who is acting as the discriminating party here. If it is a bank or any kind of payment service provider, look at the left column in that list. If it is a merchant, a company, or another public authority, then look at the right column to find the right place to report it to.
Last but not least, keep in mind that bunq offers sub-accounts with different IBANs. Not just NL, but also DE, FR and ES. If the bank that you have to deal with in your case only discriminates against Dutch bank accounts but not against for example French bank accounts, then that could be an option for you.
I've been using the NL IBANs from bunq for many years now while living outside of The Netherlands and hardly had any trouble with them, but I would imagine depending on the country that you live in the situation may be different. In the end though, it is a law throughout the EU, so there's really no excuse to be made.
