You can get it from here:

https://dl.gl-inet.com/firmware/mt300n-v2/v1/

Flash the openwrt-mt300n-v2-3.203-0805.bin

There is also a less stable but newer version you can try from this folder:

https://dl.gl-inet.com/firmware/mt300n-v2/testing/

openwrt-mt300n-v2-3.204-1112.bin

You might have to follow the debrick guide if it fails to flash: