Rules as Written: Teleportation does not count as movement
As a result, Teleportation would be able to move a creature outside the radius of compelled duel without triggering a saving throw.
Regarding the scenario where the creature is already 30' away: if they attempt to move to any location that isn't within 30' of the caster, they'd have to make a wisdom saving throw, even if they're moving towards the caster. So they would have to succeed on a Wisdom Saving Throw or be stuck in place.
At my table, I generally rule that movement towards the person who cast Compelled Duel does not trigger this saving throw, regardless of the distance, but my ruling is not the Rules-as-Written.
Rules as Intended: Teleportation cannot be used to automatically escape the radius of Compelled Duel
Contrary to my arguments in my post, the 5e designers have expressly indicated that teleportation does not count as a valid method to elide the saving throw in the 2020 edition of the Sage Advice Compendium:
Can a creature under the effect of compelled duel teleport more than 30 feet away from the caster? No. You can’t move farther than 30 feet away from the caster of compelled duel by any means, including teleportation.
At my Table: Teleportation does not work
I do maintain that the Rules-as-written support my above response: that because teleportation isn't movement, it wouldn't trigger the Saving Throw of compelled duel. However, I think it's important to consider that compelled duel is (outside of some niche exceptions) a Paladin-exclusive spell. As DM, this leads me to feel that loopholes which circumvent the capabilities of the spell should be regarded with some degree of prejudice. So at my table, I would generally side with the Rules-as-Intended, not the Rules-as-Written, and therefore argue that teleportation to escape the radius would be subject to the same Saving Throw as normal movement.