Install OEM firmware Sercomm H500

My English is bad.
I have installed the OpenWrt firmware to the Sercomm H500 router. Wifi doesn't work, and now I want to reinstall Oem Firmware. What instructions for installation?

Did you check its device page?

It's talks about bootloader based recovery, which should help you with the OEM firmware again.

Yes, I checked it. But I don't understand how to do it.