telephone repair

Good afternoon, dear Apple I am writing to you since I ran into a problem. I am not a person from a wealthy family, well, I have been using Apple products for more than 15 years and I consider them to be genetic. But a month ago, my 11th iPhone began to reboot by itself, I contacted the Apple representatives, he gave me advice, I did everything. But my phone is still rebooting exactly, I took it to the store where I took the phone so that it would be sent to the service center for checking and repairing, but nothing was fixed. I live in Europe in Ukraine and I know how coldly they can do everything (((the service center called me after 14 days and said that they hadn't found any problems. And literally today my phone rebooted back 2 times. What should I do? What should I do if service in Ukraine does not want to work (