Fusing greasy or starchy materials like flour or grease is not a good idea. This will eventually lead to a blockage. A clog like that might cause significant plumbing issues and delays in draining water.
Your sewer line could have a problem if you smell sewage inside your home. The health risks necessitate an immediate call to a plumber.
You should always hire a licensed professional to work on your plumbing, even if unlicensed individuals offer their services. The only way to know they are qualified is to put their training and expertise to the test. Make sure they have insurance as well.
A plumber can unclog pipes using several different techniques. Hydrojetting is one technique that can be used. A lengthy hose is used to "snake" through your plumbing. After that, they force water through your plumbing. It's a simple and efficient method for unclogging drains.