Hi,
the rules about dagpenge are strict, so you can not do anything about it.
You should have signed up at latest 14 days after graduation from bachelor.
The A-kasse can not make exceptions to this rule.
Another thing: Be aware that Union membership is a different thing than A-kasse membership. You (only) need to be member of A-kasse if you want to gain right to dagpenge. However, because you can not earn right as graduate, you need to earn right as wage-earner.
You can find the generel requirements here: https://www.a-kasser.dk/benefits/
Best regards,
Anders Weber, A-kasser.dk