i would suggest using banner tape.
you need a special nozzle to get the heat below the hem if using a gun.
you also need pressure rolling over it at the exact same time you apply the heat allowing it to bond while the material is hot. so your fusing it together.
the type of banner media you use will play a part too because cheaper type media will simply buckle with the heat. that’s not just with a hand held gun but with the banner welding machine. which i have but prefer the tape on general sized banners.