On this page of our guide to Sniper Elite 4 you will find out how to destroy the first cannon and how to get to the cryptology office as part of completing the sixth mission of the game - Magazzeno Facility. Both of these mission objectives can be completed one by one while remaining in the same part of the map.
The cannon in the big bunker won't be useful to you anymore (M6,4) - in the west coast, you should go there later, the cannon can't be heard. Go to the starting place - there is a small scarp on which you must climb and use the alternative path (M6,3). In front of the bunker's entrance there are two enemies that wait for your bullets. You don't have to go inside the building - use the ladder (M6,1), set the charge on the cannon and that's it (don't detonate it now!).
Now, use the left entrance to enter the bunker, then go up the stairs and get on the balcony. There, you can find an officer and radio operator patrolling the area. But you are interest only in the corridor ahead of you and the first door on the left - there you can find deciphers' center (M6,5). Go to the marked machine and interact with it. Warning - this action is a little bit noisy and can lure enemies.
Lastly, go back to the starting point. When you are on your way (you can run and use the same road) detonate the charge on the cannon.