I just purchased an ONN 4k box (not the pro) and received the 403 error when entering 250931 on the Download app. I tried your suggestion of installing the WebView App from the Downloader App. It installed ok but I am still getting the dreaded 403 error. I restarted the ONN Box but still have the same error. This was a brand new purchase from freshly restocked ONN boxes.
Make sure your device has the correct date:/time: set.
Exit out of Downloader, go back in, and try to input the code again. I did it like two or three times until it went through without doing any extra steps.
Hello, that video above was only meant for people having this problem on Fire TV devices. You should try this video first.
You need to apply any updates through Google Play on your device.