I have equipped the Ioun Stone of intellect and selected 'use' and 'attune' but it doesn't increase my intelligence by 2 (currently 18). Is there something else I need to do?

Sure - ddb.ac/characters/26957939/u6c95F

Thanks

It's not doing anything to your intelligence because you've put in an Override Score value for it - which is setting your intelligence to 18 and nothing can change it. For it to have the correct value you need to delete that override score by clicking on your Intelligence modifier on your sheet and changing it on the side-bar that will pop out. Also do the same for your Charisma because that has an override score too.

Strangely - you should have 18 in Intelligence. Your base is 15 with a +1 from your racial bonuses making it 16 - and then the Ioun Stone makes it 18. So I have to wonder why your Intelligence was overridden to be 18 when it should have been 16?

Edit:
It's also worth noting that you have a Shield equipped without having the proficiency to use it - this means you are unable to cast spells as a Wizard - which is bad.

Armor and Shields
If you wear armor that you lack proficiency with, you have disadvantage on any ability check, saving throw, or attack roll that involves Strength or Dexterity, and you can't cast spells.

Thanks very much for the help.

Our DM got us to create our characters using a custom point-buy calculator which is why I have override scores on. Didn't realise it would prevent buffs but it makes sense now. I also didn't know shield proficiency was a thing! Might have to multi-class ASAP. Thanks again!

If your stats aren't what you've put in go to your character editor and in the Abilities section choose Manual instead of point buy and put in the (base) values you should have. That's a much better long-term solution than overriding.