Not a Spanish mastiff, but I also have a mountain dog, so same livestock guardian temperament. They are bred to look after the flock, and basically do what they need to, making their own choices and using their intelligence, so yes, they are very independent. Mine had no interest in food whatsoever until I switched him to raw. I am training him with positive methods, see Ian Dunbar, or Pippa Mattinson books, which uses treats and clickers. I have to rotate treats all the time to keep interest, dried liver, sausage, even ham, cheese. The most important command to establish is the 'watch' one, as this will get him to take notice of you. Google this on YouTube to get a video demo. When my dog wants to do anything, say go out in the garden, go for a walk, eat his tea, he has to sit and look up at me. At first you get him to do this with a treat raised up to your face. Click and treat when he looks at you, then gradually lengthen the time he has to look before the click. Once he gets this, you can leave it up to him to do without any command, so if my dog wants to go out, I wait with him at the door until he sits and looks at me. Immediately he looks up, I click and open the door. if he starts to pull on his lead when we are out, I stop dead, and he has to come back to position, sit and look up at me before we continue. Sometimes you look like a crazy lady but it does work, obviously you have to show him initially what behaviour you want, by luring him back with a high value treat, but then the reward is to go forward again. After a while, with all the looking at you required, they begin to just look to you, and take notice of you. My dog is a real work in progress because he's been stuck in for 5 months due yo 3 operations, so training things like recall has been on hold.
I would say, get a good positive training book, maybe go to a dog training class, but for me, the really interesting thing about training is that it is not just about individual commands but doing activities with your dog thatbuild your relationship with your dog so you are at the really early stages. With my dog confined so much we have been doing lie down stuff like 'touch' command, left and right, finding the treat under yoghurt pots, and, as he's got more mobile, following scent trails in the garden leading to hidden treats or his dinner, all of which occupy him, make him think, and hopefully, make me more interesting to him. I do wish you luck with your dog.