error code 4000 when updating iphone to new software version

Hi, When I was trying to update one of my iphones (iPhone 8 plus) to the 11.3.1 software update I got a message saying it is unable to verify update. It said it failed since it was not longer connected to the internet but I know it is connected because I keep getting notices and alerts. I called tmoble and they asked that I plug into my windows 10 computer and connect to iTunes on my computer. I first made sure the iTunes software was updated on my computer. So I started the update on the iPhone 8 plus but now I keep getting an unknow error code 4000.

One more thing, I have not been able to connect to tmobile using the tmobile cell service. T mobile feels that I first need to update the software to find out if that is the reason why I cannot connect to the tmobile cell service but of course that has not worked either.

Sorry about the long message. Cheers