Do it yourself, find the tool that goes through the toilet, wave it with your hand and go in. You can also use the toilet suction tool to suck out the stains in the sewer.
3. If there is too much oil pollution, you can ask a special company to dredge it. When dredging, scald it with hot water, and the oil pollution will be reduced a lot.
4. Take part of the caustic soda and put it into a plastic basin, and put some hot water into the basin. After the caustic soda and water are fully dissolved, slowly pour it into the sewer.