How to increase an Ability Modifier but not the Ability Score itself?

I want to create an item that gives a +1 bump to the Charisma Ability Modifier when attuned, without increasing the CHA score itself.  I tried adding the following Modifiers to no effect:
Bonus > Chrisma Ability Checks;
Bonus > Ability Checks > CHA.

When equipped and attuned, the Modifier value on the test character stayed the same.  What am I missing?

Thanks!!

I want to create an item that gives a +1 bump to the Charisma Ability Modifier when attuned, without increasing the CHA score itself

Not possible.

I tried adding the following Modifiers to no effect:
Bonus > Chrisma Ability Checks;
Bonus > Ability Checks > CHA.

When equipped and attuned, the Modifier value on the test character stayed the same.  What am I missing?

Must have did them wrong.

Bonus > Charisma Ability Checks > Fixed Value: 1
Bonus > Charisma Saving Throws > Fixed Value: 1

Both work. Adds +1 to all skills that use Charisma and adds +1 to charisma saves. 

Bonus > Ability Checks > CHA

This would add the current Charisma Modifier to all skills. If your charisma mod is 0, then it obviously wont change. I also don't know why you're using this, it's not what you seem to want. But in any case it does work, I just tested it. 

Thanks.  By adding +1 to both CHA Ability Checks and Saving Throws (as you demonstrated) I can accomplish essentially the same thing.

Quote from Ouizel >>

Thanks.  By adding +1 to both CHA Ability Checks and Saving Throws (as you demonstrated) I can accomplish essentially the same thing.

Sort of. There are a few things that this won't do, but which you might expect to happen if you increase your Charisma modifier in general:

• It won't increase Charisma-based attack rolls
• It won't increase save DCs calculated using Charisma, such as for spells cast by Bards, Sorcerers, or Warlocks
• If you have an ability you can use "a number of times equal to your Charisma modifier", this will not give you more uses of that ability

If you don't mind my asking, why is it you want to increase the modifier without increasing the underlying ability score? I only ask because there's very little use for the Charisma ability score apart from calculating the modifier, so it might be the case that just increasing the score by +2 gets you everything you want.

Quote from wagnarokkr >>

If you don't mind my asking, why is it you want to increase the modifier without increasing the underlying ability score? I only ask because there's very little use for the Charisma ability score apart from calculating the modifier, so it might be the case that just increasing the score by +2 gets you everything you want.

You make good points about the save DCs, attack rolls, and abilities.

 I'm recreating a variant of the 1e item Rod of Beguiling.  One of the descriptors in the original is that "The rod causes its wielder to radiate an emotional and mental wave of fellow-feeling to all creatures with any intelligence whatsoever."  I figured that a bump in the Charisma modifier would be a good way to implement it because it guarantees a bump in the practical effects of charisma (as opposed to a +1 to the stat itself which, depending on the user's base stat, may not make any practical difference.)

I suppose I could build in some logic to look at the wielder's CHA and increment it until it has bumped it past the next modifier threshold, but that would take some doing, I think.  So I was hoping that the modifier was something that could be directly manipulated.

You can (hopefully) see my creation here: https://www.dndbeyond.com/magic-items/11503169-rod-of-beguiling

Quote from Ouizel >>

I suppose I could build in some logic to look at the wielder's CHA and increment it until it has bumped it past the next modifier threshold, but that would take some doing, I think.

Just increasing the ability score by 2 will do that.

Quote from wagnarokkr >>

Just increasing the ability score by 2 will do that.

 Thank you.