xfinity mobile billing

My previous xfinity promotion plan includes internet $35/month + one free mobile line. That promotion plan ended on 12/24/2025. I ended that plan before that promotion expired, and transferred out my mobile service on 12/22. I found my mobile account has an upcoming bill $44.09 for the cycle 11/25/2025-12/24/2025 due on 1/14/2026. During this cycle, the mobile service should be free. I contacted a mobile agent and she said there was nothing at their end. How can I remove that bill? I probably will transfer back to xfinity mobile later, and so I do not want that bill will be added on my future lines. Thanks.