Hi. I have dlink dir 300 c1 router and wanted to use openwrt, but cant find find this firmware for my device. Can i use openwrt for dir 300 A or B revision?
No, you can not.
Hardware specs DIR-300 C1 -> SoC = RTL8196C = Lexra architecture = not supported.
For more info on OpenWrt support status of Lexra see https://openwrt.org/docs/techref/hardware/soc/soc.realtek#lexra