Thanks for putting up with my request, I know it may seem silly but now that we know it can support it...
Is the app update you're trying to cache non i-cloud update? if so,please verify that
AllowSharedCaching is yes (should be default).
If your devices have different IP addresses (i.e. they are on different subnets) than your caching server, have you configured your dns correctly? If not take a look at this guide.
To see if that's the issue, try to connect your iPad to the same IP subnet as the caching server. If your device updates on the same subnet then you know you may need to do one of the three options listed in that article.
**if the post above has answered the question, please mark the topic as solved.