Having trouble leveling 20-30

[Niwashi]

The leveling roulette in Duty Finder is (once per day) your biggest XP source. After that, queuing for any of several dungeons will generally get you in quicker, which can be more significant than the general rule of wanting the one nearest your level. You can select up to five at a time. (After your roulette is done, though, for additional runs just select dungeons, not trials. Trials give barely any XP unless they're done through the Roulette.)

For the Novice rewards (including that ring that gives a 30% XP bonus and a +3 to every primary stat), you can either go to the Hall of the Novice in Western La Noscea (across from the entrance to Sastasha), or you can talk to the Smith NPC in the Adventurer's Guild of any main city. That unlocks the training, which can then be initiated from anywhere by selecting it in Duty Finder. (The training is solo, so always immediately available, not a group instance like everything else DF is used for, so there's no queuing involved.)