A long time ago a friend went to work for a pesticide company. They wanted to test their new fire ant concoction and he went to my place. They concluded that their new poison was no more effective than any other one. In other words, it didn't work.
Now, I dig/stir up a nest, squirt it with charcoal lighter fluid, and torch it. I do it over the course of several days. It works somewhat but they always seem to regroup.
Try boiling water. Or your lighter route, but don't disturb the mound. As high as the mound is it is also deep. The queen is at the bottom. By the time you start to make too much noise above or worse touch the mound the queen is gone. That is likely why you see the regrouping - you've made your presence known and the queen departs.
