Well, based on everything I remember from both novel and manga, he barely applied any of his knowledge. He was a soldier, yes, but military structure was superficial at best, and used mostly for edgy names. As for artifacts, instead of using modern knowledge he relied on magic. I can agree that he used magic in a new way, like making a walkie-talkie(phone?) by using earcuffs, but apart from this, I can't remember something really new and groundbreaking.
So I don't know about him using modern knowledge. I understand that some isekais don't have even this, but I don't know. It doesn't feel right for me to say he used the knowledge of his past life, especially when I compare it with works where characters at least try to use modern knowledge.