Can I use a mesh cot bumper? Baby is 7 months old and wakes because her leg has been stuck between slats.

There is very little information about the safety or use of this product.
There have been some concerns expressed about the use of mesh bumpers in general, especially if they are not fitted correctly, where there might be a risk of entrapment.
If using this product ensure it is installed exactly as instructed.

Is your baby sleeping in a sleeping bag?
Using a sleeping bag often reduces the likelihood of getting legs stuck between slats.

Parents are generally concerned about the potential for injury from the infant bumping up against the cot sides, or getting an arm or leg stuck between the slats. This may still happen even with the air mesh type products in place.

Mesh type bumpers are unlikely to reduce impact of child hitting sides of the cot, unless they are soft & padded.
Red Nose does not recommend using soft & padded bumpers in the cot.
https://rednose.com.au/article/at-what-age-can-i-introduce-cot-bumpers-and-pillows