Why does my iPhone randomly reboot then wifi greyed out

hello

my device is iPhone 6, its reboot randomly then wifi greyed out, I reboot the iPhone for a few time then wifi work again, when it happened it's reboot a lot in certain hours then it's work again for about a day,

and I have a problem with my carrier signal it shows a no service or no signal

I tried to restart the iPhone for many times its work fine for five minutes then it's back again,

I have checked the wifi flex cable it's connected correctly, and I replaced the battery it's didn't work.