This is usually not something a filter fixes. It’s more often caused by a very bright background creating flare/haze and lowering contrast.

Things to try while shooting:

• Reduce the background light relative to the subject.
• Don’t over-blow the white background more than necessary; expose so the background is only just overexposed.
• Check your histogram and bracket from darker to lighter until the white background is barely clipping.
• Increase the distance between the subject and the background.
• Make sure the lens is very clean.
• Check focus, and try a smaller aperture if appropriate.

If the background is meant to be pure white, a handheld incident meter approach can also help, but the histogram method is a practical guide.

Lens quality can matter too: lower-quality lenses and many phone cameras are more prone to reduced sharpness/contrast in this kind of setup.

In post, you may be able to improve it somewhat by boosting contrast/luminosity selectively. One community suggestion was to create a grayscale version, increase its contrast, then use it to influence the original image’s luminosity, adjusting with overlay or multiply as needed. That can help, but it won’t be as good as fixing the lighting/exposure in-camera.