failed to download ios15.4
Failed to download ios15.4 both on computer and iPhone. WIFI is connected, iTunes is on latest version, and I had checked firewall to make sure it allows iTunes. I also rebooted my phone and PC several times. I even reset my iPhone. However, those methods don't work at all. My system is ios15.3.1 and my phone is iPhone12. Please give me an hand if anyone knows what's wrong.