Okay, but you have to be more precise with your question.
There could be nothing wrong with niether of your devices and the support that you recieve from Xfinity.
If you only get a connection when connected to a LAN computer. That means that you asked Xfinity to connect your router through your computer gateway. Your LAN computer is registered as your DHCP provider. You have to call them and ask them to connect other devices using your router gateway.
Make sure this rule is confirmed with both you and the service provider Xfinity.
You are wasting time trying to troubleshoot anything else.
There are more steps to troubleshoot if in fact there are computer DHCP issues. Like netsh reset. But I seriously doubt that. In any case the netsh reset is the same as adapter properties. You are just having trouble reseting it. That's the right click disable, connect/disconnect, status, diagnose options.
A known issue. Does anything here apply ?;