Hello,
Today, i wanna cek serial number, then im searching and got "cat /proc/cpuinfo", but on my device the serialNumber not displayed
OpenWrt does not use (or care) about the serial number in any way - and there is no standardized way for vendors to store it on their devices.
If you care about it, you will have to investigate a way to extract and decode it yourself. If you're lucky, you might find it more or less unobstructed in ubootenv or at a fixed offset in the wifi calibration data or other vendor specific configuration partitions - if you're unlucky it's encoded in some less obvious way based on other hardware specifics.