Hello and good day to all

I'm new here and was hoping for some answers from you nice people. Well i live in Iceland and juste got my first frog. Its a whites tree frog. i have it in a 63L(16Gallon) "cage" and im having trouble keeping it warm. The heat here is around 19-22c° or 66-72F. Witch is to cold for the frog i have a heating mat under the cage. So heres my question what is the best method to get it hotter? should i get a light bulb for the frog? what light bulb?

other tips and tricks appreciated

In hope for a quick response best regards TraxData