If the water isn't moving, feed a manual auger into the pipe. Crank it until you feel resistance (a clog). Push and pull to break it up. Pull the snake out and clean off the gunk. Repeat.
We rarely think about our drains until something goes wrong. You know the scene: it’s a Friday night, a torrential downpour is flooding the driveway, and you look outside only to see a growing lake forming next to your foundation. The culprit? A clogged outside drain. how to clean outside drain
Set your wet/dry vac to "wet mode" (remove the filter bag). Put the hose directly over the drain opening or fit it into the pipe. Seal the gap with a wet rag. Turn it on. You will be amazed at the disgusting slurry that comes out. This is the most effective method for most homeowners. If the water isn't moving, feed a manual auger into the pipe