I am using a router called as Linksys WAP300N. My device is not supported but it's hardware information has been documented on OpenWRT website. It's SOC is Ralink RT6856F and wireless chip is RT5592N. So, my question is that can I build OpenWRT on my own as no builds of OpenWRT have been released for the Linksys WAP300N?
That would be original, low-level development - either you can do it or you can't. If you have to ask, you can't, there are no guides (there are no guides ti write a bestselling novel either).