I had run into a similar issue after I flashed the L-24 cust.img on my L-22......I did the update from update.app downloaded from the Huawei support site
http://consumer.huawei.com/pl/support/index.htm
This link is for Poland region, I used the one for India. However, in the products supported I do not see Honor 5x, so did they officially release it in Poland or did you buy from a cross border vendor? If so, you need to figure out what region your device was originally made for, search for the device firmware in the aforementioned site and download it.
Create a folder called "dload" in root of the you sdcard, copy the update.app there. Press power + Volup + Voldw for a few seconds, the update should happen. good luck.
you need stock recovery for this to work, and I am assuming that your adb drivers could be a problem...going by the "device not found" error you reported earlier
To fix it, download HiSuite and install it, it will install the all the necessary drivers for 5x as well, just to sort out the confusion