How to troubleshoot the panel only operating from Battery but not AC when a D928 is connected.

Check for proper wiring connections to the D928.
Several different panel types use the same type of removable terminal strips such as the D7112, D8112, D7212B, D9112B, 9000 series, G, GV2, GV3 and GV4. These removable terminals were intended to simplify replacing panels.
The problem may have been caused during the replacement of a D8112 where the left side terminal strip was moved to the replacement G, GV2 or GV3 panel. The dual phone line module used on the D8112 (D128, predecessor to the D928) was installed the with the black wire connected to terminal 4; earth ground on the D8112. Connecting the D928 in this manner to a G, GV2 or GV3 panel will result in the inability for the panel to run on AC while operation is normal when powered from battery.
Ensure the D928 black wire is connected to terminal 9 and not terminal 4.