My tablet is not turning on.

This is my first 'asking for help' post and also, I don't like posting because I think it is not that helpful, however i tried everything, but I'm stuck, so I did this.

My tablet (it is actually not a tablet, but a device that has a similar shape to PlayStation Vita, but with Android 2, But I'm saying on it 'tablet' because I cannot say on it android phone either, so people can understand) is from Aoutec and I read on its back from the inside 'HZ-060', so let's call it Aoutec HZ-060.

Here is a photo of an aoutec android player, but not the same model:

Here is its story:

My device was left for maybe 1 to 2 years without any touch (It was first turned on from maybe 6 or 8 years). I tried to to charge it back two years ago and it worked fine, but since the screen was broken, the touch didn't work, however it was working without the touch and I could play the games that can be played with buttons.

5 days ago, I decided to charge it back and turn it on, so I put it on charge and the red bulb was turning on which basically mean that it is charging. After I left it charging when I was asleep, I tried to turn it on when I was awake, but it didn't!

I was not hearing any clicks, too, so I knew the button was broken. So, I opened it up to check if there was some thing else broken and I saw that the power button, volume up and volume down buttons were all broken, so I bought them and soldered them and also the battery because I cut it off by mistake. And when I plugged the charger in, it turned on without any click on the power button! Also, the screen flickered for half a second at the start of the boot animation, but it didn't flicker afterwards, only at the start of boot animation every time I turned it on afterwards.

After that, I decided to plug the charger out and see if the power button that I soldered is working or not and when I plugged the charger out, the device turned off! And I tried to turn it on from the power button, but it didn't. Also, it was not normal to see it charging and the green bulb lighting without the red bulb working!

I decided to leave it turning on like this since I didn't find any problem with that.

I noticed today that the device go off, or basically the charger disconnect when I move the charger right or left (the wire).

Anyways, 2 days ago, after I soldered every part that needed soldering, I turned it on to see if it is still working or not and it was working fine, so I put everything together and the plastic front and back with the buttons I click from the outside. And when i tried to turn it on (by the charger), but it didn't! I thought the problem was that I broke something when I put the front and back together, but everything seemed fine. After that, I found out after some researching that the battery might be dead, or the system was frozen. So, I tried to soft reset it by clicking on the power button for 1 minute and after that when I plugged the charger back, it worked!! But as my usual, I plugged the charger out and in, again, to make sure it is working not the last time I will see it turning on, but this time it also didn't turn on. I tried to soft reset it by clicking on the power button for 1 minute again, but it didn't work also. I tried another soft resetting method by clicking on the power button and the volume up button together, but also, just the green bulb lighting, the screen doesn't show anything.

So, after that I decided to buy another battery and I did and also soldered it, but nothing happens, too.

I'm too confused and anxious that the device is dead or something and I won't be able to restore the valuable data on it again. Also, don't tell me to reset it because the reason of fixing it is to actually move the data from it, to my phone through Bluetooth.

Please, tell me what should I do to turn it back on.