If your shower drain has a screen or other cover on it use a screw driver or other tool to remove the screen or other cover from the drain.
Then pour some baking soda and vinegar into the drain and let it sit for around 15 minutes until it fizzes.
While the baking soda and vinegar are fizzing take a pot and boil some water in it and make sure to get it boiling hot on the stove.
After the 15 minutes have passed you can then pour the hot boiling water down the drain to clear the shower drain of the baking soda and vinegar.
If the smell still hasn't gone away you may have to repeat the above steps with the baking soda and vinegar but this time leave the baking soda and vinegar in the shower drain for up to an hour.
Then after the hour has passed you can then pour boiling hot water down the shower drain again and the smell should be gone now.
If not then you may have some residue of sewage somewhere in the shower drain that needs cleaned even more or something down the sewer pipe further down that needs removed or cleaned out.