Static DNS entries can be set within the modem/router or on the customer’s computers. Please check these if the customer is experiencing a DNS issue.
To determine if the customer is having a DNS issue try bringing up a website by IP address. For example if they type in this IP it should bring up Screen Connect as a test: 163.182.253.81
If it comes up by IP then it is for sure a DNS issue and either their modem/router or computer will have static IP addresses entered for the DNS servers. These need to be removed to correct the problem.