European stocks and double taxation

I am from Croatia and I have bought and received a dividend from a german stock.

I have realized that tge witholding TAX is 26% even though Croatia and Germany have a TAX treaty and I should be taxed 15%.

I have contected IBKR and they have basically told me that there is nothing I can do about it.

This is the response I got from IBKR support:

''' For Non-US products, we request you to kindly refer to the prospectus of the security, contact the issuer of the security, and/or consult a tax professional regarding the tax consequences of an investment in particular securities. IBKR might act as a withholding agent and apply the applicable treaty rates, or the tax might be applied by our clearing agent prior to receipt. In such a case, holders might be unable to take advantage of any country specific rates which may exist.

The withholding is applied at source (at the depository) and not by Interactive Brokers (IBKR). IBKR simply passes through the withholding to its clients. IBKR is unable to make client-specific election in this situation; therefore, we elect to receive dividends paid at one withholding tax rate that covers our entire client base, called a blanket election. We do not offer a reduced treaty rate or exemption for eligible clients. If clients are entitled to a reduced withholding tax rate or exempt from withholding, they should consult their tax advisors on how to reclaim the excess tax withheld. IBKR does assist on providing tax vouchers (chargeable) for specific European dividends on request. '''

Is there a way I can exercise my right for the double taxation reduction?