The larger the dent the more likely that the tube is out of round in the immediate area. The typical approach is to use tubing blocks and the frame in a bench vise. Clamp in place, maybe some grease and work it back and forth to help roll the dent out. It will not fully return, the paint will be most likely toast in that region, then fill what is left. Almost any filler will require that area to be down to bare metal to adhere. I have used JB Weld and another time body lead to do the task. Bondo only for pin hole craters. And for that use Evercoat Featherfill, it is much finer grained than body shop typical bondo, sands fast, less filling.