Greetings. I'm trying to create a magic item that will have a higher save DC for non-casters so they're on par with casters. What I want to do is 8 + prof bonus + WIS. However, I can't get above 14. The character I'm using as a guinea pig is a level 20 monk with a wisdom of 18 (+4). I've tried the "Bonus --> Spell Save DC" but it doesn't work for non-casters. I might be SOL but I wanted to ask. Help please! Thank you.

https://www.dndbeyond.com/magic-items/2086278-krukna-the-water-fathers-knuckledusters

You mean to increase the Ki Save DC? That isn’t possible. Note, that modifier you are using says “spell save,” not “save DC.” It only works on spell save DCs. However, the KI save DC should already automatically be 8 + Wis + PB. That’s RAW.

I'm not trying to change the Ki save DC. Just the DC for the spells given by the magic item.

EDIT: also, noted. Thanks.

Ohh, I understand now.

Magic items automatically pull the spellcasting ability from the PC. Since non-spellcasters have no spellcasting ability it will only ever be a maximum of 8 + PB. There is no way to designate a spellcasting ability using a magic item. That’s why most official items that grant spells are only attunable by spellcasters, the ones that aren’t typically don’t care about a spellcasting ability (like the wand of magic missiles or a wand of secrets). 

Thanks. I've decided to just give a base save DC of 18 for now.

FYI: For magic items that have a fixed save DC, the DC is typically determined by the rarity of the item. A DC of 18 would be between Very Rare and Legendary/Artifact rarity. Uncommon (like a wand of entangle)is DC 13, Rare (like a wand of fireballs) is DC 15, Very Rare (like a wand of polymorph) is DC 17. 

Thanks.