Is possible install the ax1800 firmware on ax5 (china version)?

these two routers are the same? is possible or i will brick the router?

It's never a good idea to install firmware for another router.

The possibility of bricking your router is high.

i know, there is for ax5 a openwrt firmware?

"AX" is simply a designation for a WiFi 6 device.

This is the the list of AX devices currently supported in OpenWrt...

this is the router, also named redmi ax5

As you can see in the link I posted, it is not supported.