How can I improve this sentence?
This section describes how to troubleshoot the fault concerning the disruption of all Internet access services caused by conflict of gateway IP address.
No English teacher could help you with this....however, fortunately I'm not an English teacher, I'm a computer consultant...
What you are introducing is a set of instructions for people who have set up their system incorrectly. The heading should be:
What to do if your Gateway IP addresses have been set up incorrectly
A gateway handles all the addresses which are not local to your immediate network. This would normally be the IP address of the router which gives you access to the internet.