@lightningow 

Hi,

Could you please post a screenshot of the screen where you configure the Internet connection? (make sure you remove any personal information that is visible)

  @woozle 

thanks for the answer, sorry I only have these screenshots, I dont have access to the device right now, I sent you them in private

The only settings that is changing after reboot randomly is only the username in "ETH Internet" (I pointed at it in the screenshot), the password stays untouched.
"DSL Internet" and literally everything else stays untouched. I know, it doesn't make any sense.

  @lightningow 

Since the "OK" button is in the same place where other router models have the "Save" button, let's just assume that this button acts like the "Save" button.

Here are couple of things you could try. 

Go to the menu "Advanced" > "System Tools" and check whether your router model has an entry "CWMP Settings". In case it has and "CWMP" happens to be switched on, then turn it off.

If under the menu "ETH Internet" you have the abilty to "Add" and "Delete" interfaces, then delete the existing interface and afterwards add it anew.

I don't know if it's possible to downgrade the firmware. Someone from TP-Link should know.

  @woozle thanks very much! I'll try and let you know as soon as possible! And if it doesn't work and I'll find another solution I'll will let you know aswell.

Thanks!