Question about Corsair Obsidian 500D RGB

I assume you are referring to the XR5 360-MM radiator that comes as part of the iCUE XH305i bundle. The case supports up to 360mm fans, so this should actually fit. However, if you look at the specifications of the case on the manufacturer's website, the supported Corsair radiators are explicitly listed, of which yours is not part.

My guess is that the case is intended for AIOs and not for custom loops. I would therefore assume that your radiator is too long at 39.6cm, as my H115i AIO with 36cm already takes up the entire mounting bracket.