If you have already replaced the heater and t-stat and not solved the issue, it's the board. You can test it to be positive as follows.

Unplug the unit, take the cover off the board in the back.

Unplug the 3 wire connector with large spade clips. It looks like a box almost and slides onto the board connectors from the side, not straight on.

Look at the board and identify the wires on the unplugged connector by the writing on the board. There is "line", "def" and "comp".

On the unplugged connector, put a jumper wire between "line" and "def". Make sure it's an insulated wire and not touching anything that will short it out.

Plug it in. Within a few minutes the heater should be good and hot. May even be able to see it glow. Should heard hissing. Remove rear panel of freezer to confirm it's on.

As long as the heater comes on, you need a new control board. If it does not come on, repost and we can work thru it.