Nainggolan was controversially omitted from Belgium's World Cup squad last summer.
But he revealed, “I didn't miss a single game played in Russia, it was incredible, an excellent result. I have always supported my teammates and I always will, but I would've liked to have been part of the national team.
“I would have liked an explanation, but no-one ever came to tell me why I was no longer being called up and that bothered me.
“I think it's all very sad, but it's important to me to be a man of my word. I said I wouldn't play for Belgium again and I kept my word."