Tax refund for excess witholding deducted for non resident

Hi - we have just got our residency card after waiting for over 2+ years ( yaay). We are invested in Portugal yield fund and bank with millennium bcp. The bank has been charging a witholding tax of 10% despite all documentation that we are non residents and as the fund is fully invested and paying a regular dividend the amount has now become significant.

I am told there is a way to get a refund of the same ( going back UpTo 4 years). Anyone having done that or know how that can be done?