Destroying the first cannon
After killing the officer it is worth to go on the floor in the small country house, a few meters in the north (M5,2). There, you can find suppressed ammo and TNT - can be used to destroy a cannon.
Now, go through the buildings from the north to the west where the cannon is (M5,5). It is guarded by three soldiers but additional two can walk nearby. Get closer to the cannon by using mined trench (3 teller mines) (M5,4). Get closer to the cannon so you can throw the TNT with delay-action fuse and hide in the south trench.
Recapturing the radio station
The radio station is not an easy target (M5,7). You should get on the rock from the north (M5,6) where you can safely tag your targets - there is six of them + two beneath you, one of them is the radio operator. It is best to use suppressed ammo and eliminate all of them because your goal is to get inside, on the floor. Try to position yourself so you can shoot downwind.
When you eliminate everyone get inside the building, go on the floor and use the radio station. Now try to go back at the rock from where you must observe the incoming transport (you have 2 minutes till it arrives).
You must tag all vehicles, that's it. But, the mission is not yet completed - the last thing that you must do is to ring the bell on belfry, but this is done later.