After over 2 hrs. with Hannspree and AT&T on the phone yesterday, trying a number of codes that were supposed to work.  An AT&T tech did an onsite check of the situation this a.m.  Great guy.

The issue is that you need to have the NEWER BLACK AT&T REMOTE, not the silver one.  This newer remote is accompanied by a codes sheet that officially gives 1482 as the code for Hannspree.

AT&T remote now does it all -- turns on, off, and all AT&T related things work as should - at least for the ST289MUB