Flashing "KuWfi CPF903" with OpenWrt

I just got an outdoor 4G Wireless router:

Hardware Version: CPF903-V5.0_LED_190711

It is working great! Only issue is that it listens on public interface port 80, which is horribly insecure. I couldn't find a way to disable that, so I was wondering to flash an OpenWrt on it.

The question: How can I do this?

So there is a page:

where it would let me upload new firmwares, ex.: https://downloads.openwrt.org/releases/18.06.5/targets/ar71xx/generic/openwrt-18.06.5-ar71xx-generic-ap147-010-squashfs-sysupgrade.bin

But it always says: "File selected is wrong,please select a new one again!"

My purpose is to have an outdoor router with 4G connection, which I can share via Wifi.

Really many thanks!

@kuwifi, welcome to the community!

Have you verified that this device is supported?

I don't see your device listed in the Table of Hardware.