The Shikan plains might seem like a ginormous place but the number of big settlements was less than a few dozen. Countless monsters roamed all over the Shikan plains, and it was not necessary for Dilan to do anything against them to unite the continent.
Sweeping through the settlements with the Red Dragon made things a lot easier because everyone was given a 'pleasant' demonstration of the Mythical creature's dragon breath before they were given the chance to surrender or to fight Dilan and the Red Dragon.
It was only obvious that the humans surrendered to Dilan at the sight of a several hundred meter long flamethrowing Dragon. If Dilan wouldn't be as powerful as he was right now, even he would have to succumb to the Red Dragon just like the Ascenders before him who were presented with the choice between surrendering or dying.