If normal lens cleaning fluid doesn’t affect the marks, they’re likely scratches or coating damage rather than dirt. In that case, more aggressive cleaning is unlikely to help and may make it worse. A lens pen is about the only mild alternative worth trying, but use it carefully.

Because the damage is on the rear element, it can affect image quality more than similar marks on the front element. If the lens is newly purchased and returnable, returning it is the safest option.

If you’re considering keeping it, test it first in demanding situations: high-contrast scenes, bright backlighting, and areas where flare or loss of sharpness would show up clearly. If the fogging is visible in real images and bothers you, the practical answer is replacement/return rather than further cleaning.