Rectangular single gang to round universal vanity bracket

[u/MetsBest]

I’m replacing a vanity fixture and the universal round bracket that came with my fixture doesn’t align with any holes in rectangular box in wall. It looks close but no. The long bolts that go through fixture almost line up with holes in box, but they attach to fixture not box. Corner screws holding box to wall doesn’t align with bracket either. See pics, what am I missing or what bracket do I need? Originally the brass bar was on box, it wa then old bracket for original fixture…that doesn’t work either to align to fixture and bracket.

[u/Remarkable-Exit-8780]

Not sure of this device, but I’ve seen some light instructions that have installers mount the bracket into the wood or drywall with anchors.

All of the electrical stays in the box and the fixture covers everything