External hard drive for ipad /icloud

No, the charging cable will not work. Depending on the iPad model you are using, this adapter may enable you to connect an external drive to your iPad. Also, be informed, that some external drives will need an external power source and the iPad may not be able to sufficiently power it.

Also, you may be to read and copy certain data from the external drive over to the iPad and maybe vice-versa. But, generally speaking, a backup of the iPad is not done this way.

To know more about connecting external devices to your iPad:

Connect external devices or servers with Files on iPad - Apple Support

To know more about back up:

Back up iPad - Apple Support

You mentioned iCloud storage. Verify that you have enough storage and that your app is setup to be able to upload/save data to iCloud. Also, be patient sometimes it does take time to complete the upload, depending on the speed of your internet connection, etc.

Manage your iCloud storage - Apple Support

Here is the support page that details the available methods of backing-up your iPad:

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

System backup to a connected Hard Disk Drive (HDD) is not supported - but you can manually copy files to an external storage device if you wish to do so.