To use your Afrihost mobile data ensure you are using the Afrihost APN.
The APN should be set on the device you wish to use your Afrihost mobile data.
From your screen shot we can see that you are using the (mymtn) apn which is why your airtime is being depleted.
Afrihost APN = afrihost
Ensure all letters are lowercase and there is NO space before or after afrihost.
Certain mobile devices revert to old or default Access Point Name (APN) settings after a firmware or operating system update, so please make sure that your Access Point Name is still set to afrihost, and not myMTN (or another setting).
Afrihost have a great web page to help you set the correct APN settings for your device (http://www.afrihost.com/landing/mobile/apn/).