iRig Stomp I/O controls AmpliTube as well as functioning as a superb audio interface for it. Since you appear to be having technical difficulties I'd suggest opening a ticket with IK Support at
http://www.ikmultimedia.com/contact-support if you haven't done so already so they can assist you directly and officially and get you up and running.
It seems you may have an open ticket so in that case I'd suggest telling the support rep that the suggestions aren't working as I know people use iRig Stomp I/O with AmpliTube every day without issue and I'm sure they can get you to that point too.
Edit - you did have a ticket and there was a USB-C cable that works suggested to you and you should try that and let them know if that works as it appears the team has tested that specific cable with positive results in controlling AmpliTube for newer iPads.
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