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?

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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.