The bright center and darker corners you showed in the image is lens vignetting.
The circle may be worsened by a filter. Interference filters permit light to enter perpendicularly, so you can get artifacts around the edges. The issue may be a combination of the filter and vignette.
For now you can manually correct for this in Lightroom: raise the corners, lower the exposure overall, crush the shadows, and increase the contrast until the sky appears even. There is also specialty software for this correction as many lenses suffer from it.
Next time try stopping down the lens one or two stops and make sure lens corrections are turned off.