Playing as the Germans I've noticed allied AT guns do not engage infantry.. I'll have infantry in a building within range and LOS advertising to all their presence and the AT guns will do nothing. Move a vehicle alongside and then they'll open fire. Played the game before and I don't recall this being an issue.

In the weapons.adb file which contains the configuration for all the different weapons there is a setting for what are vaild targets for a weapon. Just looking at the 57mm AT gun it is set to [6] "Vehicles and Terrain" as valid targets. Their primary target is set as vehicles as well. You can adjust the files to make all targets viable if you wish. They are still likely to prioritise vehicles and unless they have an HE shell loaded I'd imagine they are going to be reluctant to target infantry with AP rounds.