Almost no one is going to be productive the first few months they step in to working on a massive codebase. In a large codebase you essentially have to develop a relationship with it and the teams working on it to have an understanding. Find simple tasks you can accomplish even if it's outside your current scope just so you can get used to making changes.

Don't be afraid to switch roles though if 6 months in you still don't enjoy the work. It's not a necessity to work on large codebases.