To increase your Player Level cap, you must Ascend your World Level. You should have a World Ascension Quest in your quest log which will ask you to complete a dungeon. Once you complete this dungeon, your world level will be ascended and your player level will continue to increase again. Increasing your World Level increases the difficulty of enemies you encounter, but the Coin and XP those enemies drop is increased as well!
**Note that any XP you earn while capped should carry over once you unlock the new World Level

For example:

If you cannot move from level 19 to level 20, you need to increase your World Level. World Level can be increased by completing a challenge that is accessible Via a quest with Syl'aire in the Velvet Shroud. You will need to complete a dungeon and if successful, their World Level will increase and the Player level cap will be increased.

Check out our video for more on how to increase your level cap to level 20 and beyond: