If --ar appears to do nothing, one of these five things is happening.
1. Syntax
The parameter needs a space before it and a colon between whole numbers: a lighthouse --ar 16:9. Common breakages include --ar16:9, --ar 16/9, and a smart dash pasted from a word processor. Retype the two hyphens manually.
2. It must be at the end
Parameters belong after all prompt text. Text placed after --ar is frequently discarded.
3. Version limits
Older model versions clamp extreme ratios. --ar 21:9 works on V6 and V7; on legacy versions it is rounded toward a supported ratio. Add --v 7 to be sure.
4. Non-integer values
--ar 1.91:1 is rejected. Use the nearest whole-number pair, such as --ar 191:100.
5. Upscaling changes the frame
Some upscale and zoom actions re-frame the image. Use Custom Zoom to set the ratio again during the upscale rather than expecting the original --ar to persist.
Quick test
Run a red cube --ar 16:9 --v 7. If that returns a widescreen image, your syntax was the problem in the original prompt.