If there's an IP address conflict then you need to identify all the devices that are connecting on the same subnet, and find out what IP addresses they are using.
The most likely reason for an IP address conflict is that some or all of the devices are using fixed IP addresses.
If you use DHCP and someone then assigns themselves a fixed IP address, then there'll be a clash. If you use fixed IP addresses without tight control over what device uses what address, then you're likely to get an IP address conflict.
Best to use DHCP with an address pool large enough to supply addresses to all devices on the subnet, and not use fixed addresses.
Another possibility is if you've got more than one router / wifi access point acting as a DHCP server on the same subnet. Carnage.
DNS has nothing to do with your problem.
SD