Ok, I've solved the problem, I got in contact with a certified Roland technician and this is how you solve the problem.
But first, I must explain why did I need to make this change and not the technician? That is because I bought my Fantom G7 second hand so no warranty applied, and my country doesn't have a certified Roland service. There is a Roland certified service in another country, in a city about 600 km far away from mine, so I couldn't get my keyboard over the border without paying some taxes, and the trip itself would cost me. So my friend was visiting that town where the service is, and bought the motherboard and brought it to me. I took the motherboard and my keyboard and brought them to an electronic technician who just replaced the motherboard, he has no knowledge about the software.
Initially, when you replace the motherboard, your Roland "thinks" he is a G6 by default. What you have to do is enter the Service test mode, and you enter that mode like holding down the TEMPO and EFFECTS (ROUTING) keys while powering up your Fantom G. When you've entered the service test mode, you go to the number 25 which says Keyboard. There you will see that it stands G6 on the display. All you have to do is press SHIFT and SOLO SYNTH together, and G7 will come up.
Next, you press SHIFT and EXIT and go to factory reset, and when it is complete, you power down, and power up your keyboard.
I hope it will help somebody.