Residence Proof (EU certificate) - Never Applied (Vienna)

Hi everyone,
Been living in Austria for the past 3 years doing a PhD and currently writing my thesis (unemployed). Trying to claim some help for my monthly rent as it is quite expensive on social welfare and I was requested to give residency proof. I've already given them by meldezettel and just noticed that I should have registered for an 'EU' certificate 3 years ago. I can't register now as it's too late and I'm also now unemployed....any advice you can give me as to what to do?