I use my midi keyboard only in Chordelius game, in order to effectively learn all those chords I still don't know (which also helps me with the game...)

Since this is ear training, using an instrument is cheating your ear (and cheating at the game, no doubt, but your ear is more important, musically, than the game and your TPI.)
But if you do it after each stage (hit the C key, then try whatever buttons you like), along with the piano, especially for chords/scales/intervals you're having troubled with, I can't see how that wouldn't be helpful. After you do that, hit the C key again to get to the next stage.

Okay, thank you guys.. By the way, I just purchased the Steinmay Grand Piano but whenever I choose the option to use it before each game, it always comes back to the default piano when the game starts. Is there any way to solve this? In addition, what do you guys think on how to be better at the games and to progress faster?

As @John Miller says, this is ear training and I believe the best way to improve the skills involved is doing your daily training, maybe adding some other exercises. I have experienced that "faster" and "better" do not match very well in these learning proccesses, but everyone has his/her own pace.

I wondered the same thing with the instrument resetting - it's because it gets randomised each game. You can manually select the instrument below, before each game, if you really want to. I did this for a while, but then gave up, and actually found that the changing sounds is better for the training, because your ear has to work a bit harder to adapt to the different timbres :)

Yes, but whenever I pick an instrument, it is not randomized but it keeps coming back to the default instrument. Is this a bug or I'm doing something wrong? @ToneGym Community

Do the regular piano and Steinmay sound exactly the same? I don't hear a significant difference.

Listening more closely, they sound the same but the Steinmay is a bit louder.

I wouldn’t say, that it’s cheating to use an instrument. I just think that you just learn a slightly different skill. That is creating a certain chord or scale instantly on an instrument. But this also depends on your skill level on that instrument. I am a lousy piano player and although I’ve made big improvements on chordelius it takes me „ages“ to figure a chord I recognised out on the keys. Vice versa you might struggle to recognise chords solely be ear. But I guess, that it won’t take to long to learn the „other“ skill once you mastered one of them. This is clearly just my experience with learning any musical skill and not by any means scientific…

For example you might experience, that you are a bit worse at a game when you switch to an unfamiliar sound/instrument. But that you‘ll progress really fast to be equally good with the new sound as with the old one (is this proper English?).

Sorry for the spam: For me that means, that you first and foremost learn exactly want you‘re doing (recognising a chord or interval with a very specific sound and when you‘ve got options to choose from). Nonetheless, the better you get, the faster/better you‘ll adapt to similar tasks.

My thoughts are use an instrument but make sure you do a daily training which is longer than the basic five games basic training, over time as you will get familiar with the games you'll be able to drop the need for an instrument it will happen naturally.

With regard to choosing another "instrument" as a default, its in the settings, there is a switch there I think under "training preferences"