There are 3 ways to view image of cameras on DVR system.
1. View images on monitor doesn't need Internet. So a router is not necessary for view on monitor.
2. If you want to view on computer and phone, you need Internet. So a router is necessary for view on computer and phone.
But please notice system should not be connected to computer or phone directly by cable. System could be plugged into router to connect Internet by ethernet cable. Phone and computer is also connected to Internet. Finally, phone and computer can use Internet to connect system.