Phone restarting for some days than died

Hi everyone, at first I want to apologize for my bad english and to thank you for your time.

Last year I brought my uncle’s dead iphone (won’t turn on) to the apple store and after replacing the battery they gave it to me saying that the motherboard was dead and that i can keep the new battery. So far so good I bought a new motherboard with touch id, a new Charging chip (just to be sure) and i swapped them. The phone worked fine for 1 year and a week ago it started to restart in loop while i was using it, The problem was so bad that I had to reset the phone with itunes. Nothing changed and for the last week the problem persisted. Until yesterday: i was using safari and suddently the phone shut down without even restart. I tried to charge it, i tried to restart in every way but it’s dead.

Now I’m asking you: do i have to buy another motherboard? Or can I do something cheaper like test and swap some capacitors? In that case i have a tester but i don’t know which capacitors I have to test and which result indicates a bad one. And last question: could it be that I installed the motherboard in a wrong way? It seems strange to me because if I failed with the installation the phone couln’t work for 1 year..

thank you again for your time, I really appreciate it!