Use a toilet auger (not a regular drain snake). Insert it past the P-trap and crank gently. If you feel resistance, you might have found a local clog.
Look for a white PVC or cast-iron pipe cap with a square knob outside your house, near the bathroom, or in the basement. Pro tip: open it slowly, and have a bucket ready. 🪣 how to unclog toilet main line
Feed the snake toward the street/main sewer. When you hit the clog, crank and pull back debris. Repeat until water flows freely. Use a toilet auger (not a regular drain snake)