I understand your frustration. You are using the New Smartthings App and not the Classic App. The so called “new” App is the future with plenty steps backward and full of bugs, what turns up time to time and makes people’s life miserable.
So you paired the device. And you can do press and hold for both buttons. That gives 4 options to do things. As Smartthings allows you to interface different devices it gives you the optuon to define your actions for those options. It can be lights, scene activation, turning up the heating on your thermostats or opening blinds, anything. Sounds good? But you want the feature how IKEA sells it. To brighten the light.
Version A, you can pair it directly to up to 10 Tradfri bulbs according to IKEA’s description. Then you can use the basic features, turn on/off for press, brighten/dim for hold. And you don’t need to define any action in the Apps. If you do the pairing correctly then you need to hold the button on the back for 10 seconds close to the Tradfri bulb you want to pair. Then you are finished. Your remote will do the work. But I guess you want to control other lights than Tradfri ones. I have a Hue system and LIFX bulbs too. You cannot pair with them. Here comes in Smartthings solution to control anything.
Version B, the disappointing thing, as out of the box you cannot just increase brightness. You can set by actions preset brightness or color etc, but you cannot increase by 5%. Here comes in the Classic App. In the Classic app and through the ide you can install the SmartApp called ABC, advanced button controller which has a lot of good features and one of them is to brighten or dim by x %. When you press you can set 5 % and if you press again and again then it changes by that percent step by step. Not as good as IKEA’s direct control but does the job.
Version C, other methods like WebCoRE or Sharptools.io, you can define your desired action through does rule engines.
I hope you can find a solution what you will like.