Fix my network DNS problems

Small business with 5 users a BT business internet connection, cisco 4300 router, draytek router. Connecting to the wifi or wired network internet often throws up DNS errors and patchy connection. Our mobikle phones connect wsithout any problem at all. Laptops are relatively new and connect fine on home networks. The problem seems to be something in the office that affects the laptops but not the mobiles. I need someone with the knowledge and skill to troubleshoot and resolve the problem.