Sidaner's English language Wikipedia entry is brief, so consider the more detailed French entry and have it translate the text if you don't read French. A useful article about him from 2014 can be linked here on the (British) Spectator site.
As for his style, Sidaner's divisionism was a treatment he applied much of the time to fairly structured draftsmanship. That is, if you look at the thumbnail images at the Athaenium site above, many seem sharp. It's only when you link through to enlarged versions that the brushwork becomes more dominant.
An example of Sidaner's early style and subject matter.
Divisionism is beginning to creep in here in the form of small color areas. The arms and hands are poorly done. In any case, he seldom painted people.
A pleasant scene: one of his more interesting works with a Symbolist touch.
Le Sidaner avoided Paris, spending time at various places in France and Belgium. A few of his works deal with the London area.
Note the small, lighted window. This makes the image less relentless. Examples of this can be found in a number of his paintings.
Two lighted windows here (plus one reflected window) in this pleasantly moody painting.
He often included tables with a bit of still life as subjects. Here too is a lighted window.
Another Sidaner trait was showing scenes beyond open windows.
A fairly late painting. Color areas return, though some divisionism remains.
Painted not long before his death. Atypical subject matter. More of a sketch than a finished work (in Le Sidiner's sense).