There are a few spells that the DCs are not the standard calculation; Dismissal, Banishment and Knock are in this category.
Knock DCs are based upon
1. Caster Level
2. Your Caster's Primary Attribute Modifier
Each Caster Level you gain is a +1 to the DC. Arcane Augmentation can be used to boost your Caster Level See
Arcane Augmentation Items
Each additional Primary Attribute Modifier is a +1 to the DC (Or +2 to the attribute)
10th Level
1d20 + 10 CL + 9 mod (22 base stat + 6 item) = 20 - 39
You will be able to get most locks for Base 8 quests at the Elite Difficulty (Level 10) - You may need to roll a few times. See "Traps and Locks" link below.
With Tomes and Arcane Augmentation and Past Lives the numbers get higher.
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One of the main reasons using wands/scrolls of knock are limited is because they are considered cast from the lowest possible.
Scrolls and standard knock wands cast at Caster Level 3, with a 1 modifier (12 attribute is the minimum required for a level 2 spell)
This would result in -> 1d20 + 3, this will get low level chests as long as the lock DC is less than 24
For information on what DC is needed there is this resource on DDO Wiki
Traps and Locks
Knock can work through most heroics with some outliers where the DCs are high (many of these have a key which is why they are high). Epics there are more locks that will exceed the Caster's Knock potential, but these are difficult by design and require a dedicated trapper well geared.
My opinion, Knock is worth having if you don't have any other option to open locks.