I have a background in military communications, and, using an Omni-directional antenna, especially with all of those obstacles just will not do it. You will need to set up rely point throughout your plant.
Use a WIFI finder, a smartphone will do, and find the strong and weak points in your structure. Pinpoint them on a diagram of the plant and from there, setup your relay equipment to cover the weak points.
Did this many times setting up retrains sites in the field. Need to cover everything, even if it means getting more equipment. An Omni-directional antenna was never that good of an antenna.