this option isn't available in my default backup app ( huawei backup ) android 5.1
note: the managers category doesn't contain any wifi info
If your backup app doesn't have such a category, then you cannot back it up. Even in cases where it is synced to your Google account (via the Settings > Google > Backup option) , it is restored only to the phone when you restore the backup while setting up as a new phone. But that will not show you the raw credentials if you're looking to use them. Do note that Google 's backup has its own flaws and this restore may not always work. If you need the raw credentials, then you will need to ask the owner of that WiFi network.