openwrt – Is there a way to upload firmware to router via uart?

yesterday i bought mercusys mr90x and i was trying to install openwrt on it.

I connected to the router via ssh and copied firmware bin file to the router and executed

mtd -e mtd2 -r write /tmp/fw.bin ubi0

this command. Then it uploaded firmware and rebooted the router. But after all of this steps i couldnt access to the router. Now i need to reupload original mercusys firmware by using uart.

I searched how to flash the router from tx rx pins but i couldnt find anything. Is there a way to install original firmware to the router ?