First you need to go to Village of Thule. You need to get to seacret underground passage in the Field Of Lava. It looks like usual broken tree. It is guarded by Fire dragon and several firebirds that are stick to that location. So you need to be in shape to fight them in order to complete this quest.After entering Passage you need to open the door with the governor's Key. The door is guarded by ice dragon. So be in shape to defeat him too. After opening the door you can see another portal there that is deactivated. You need to close the door to the island of fire to make the portal to the island of Ice work. Do it by simply tapping the door.
You will appear near the Village of Thule. On the ice island.