Need Help Negotiating an Offer

Hi all,

I accepted a job offer (as SWE in Germany) recently without almost any negotiation. I was told I can do so later with the company management. I didn't start yet, but it is controlling me that I didn't negotiate, and accepted a kinda low ball offer. What do you advice? Is there a possibility to get this negotiated later in a salary review? Or if I get promoted, does it mean putting me in the bracket of the next title (which might fix it)?