Here's a free link to a manual.
1. The procedure for switching modes is a little complicated, see the instructions if you don't already have a manual.
2. Has it ever worked on DC? It will take several hours from room temp to start to show cold in the freezer, maybe as long as overnight
3. Does it work OK on 120VAC and LP?
4. There is a terminal block on the back for DC input. Does it have 12 VDC across those terminals?
Update us when you check out the above.