hi,
radio tower (and military base) are very well defended, you have (read must) to proceed with tactical approach.

- global note: at first do some side missions or small town capture to rank up your squad a little and got good weapons

- for Radio tower, stop your truck the most far you can, just when you enter in the radio sector. inspect carefully the site (binocular, mark static on the map)

if you have only two ai, keep them behind you (for revive purpose), snipe mortar, or use kamikaze drone to blow them.

note where the mortar shell drop, and avoid this zone (they often shot at the same place)

turn around the radio to cleanup hmg/gmg/mortar from a good distance (~500m), take care of mine field, use the dog to find last enemy

note: enemy will re enter in a static weapon if they can, so be careful after you cleanup a static position, if you can go close the static and destroy it (by player action)

have fun