I imported my model from Solidworks with urdfconverter. The robot looks fine in RViz, but in the Gazebo, it looks submerged, and the prismatic joint doesn't work well, making the table fall. Also, the fixed joint doesn't appear. During starting, the robot moves.

Here is my urdf file.

For these problems, I searched, and one said, "try to correct the inertia." I tried to correct them, but with nothing happened. For the fixed joint, I found that it is better to put it revolute with limits 0, but this transform makes RViz collapse. I am using ROS melodic and gazebo 9.