The Dimensional Shackles state:
You can use an action to place these shackles on an incapacitated creature. The shackles adjust to fit a creature of Small to Large size. In addition to serving as mundane manacles, the shackles prevent a creature bound by them from using any method of extradimensional movement, including teleportation or travel to a different plane of existence. They don't prevent the creature from passing through an interdimensional portal.
You and any creature you designate when you use the shackles can use an action to remove them. Once every 30 days, the bound creature can make a DC 30 Strength (Athletics) check. On a success, the creature breaks free and destroys the shackles.
Does the bolded text mean that they actually have all the properties of regular Manacles?:
These metal restraints can bind a Small or Medium creature. Escaping the manacles requires a successful DC 20 Dexterity check. Breaking them requires a successful DC 20 Strength check. Each set of manacles comes with one key. Without the key, a creature proficient with thieves' tools can pick the manacles' lock with a successful DC 15 Dexterity check. Manacles have 15 hit points.
In particular then:
- Can Dimensional Shackles be escaped with a DC 20 Dexterity Check?
- Can you break them with a DC 20 Strength check?
- Can somebody else break them with a DC 20 Strength check? (The DC 30 check only applies to the bound creature, unlike with Manacles, where any creature may attempt to break them)
- Do they come with a key?
- Can they be opened with a DC 15 Dexterity Check using Thieves' Tools?
- Do they have 15 hit points?
For example, if they only have 15 hit points, couldn't you simply headbutt them until they broke, which would take far less than 30 days.