Dvr networking problem

[Satnam_2]

Internet stops working immediately when I connect dvr and modem through lan cable . I cannot find where is the problem, is it in port forwarding or something else. I have changed lan cable . I have put dvr ip address in dmz and also put dvr port in virtual server . Something is conflicting in the network .

 

[DeauteratedDog]

About the modem: is it just a modem or is it a modem/router combo?
Is the DVR setup with a static or dynamic (DHCP) IP address?
Is there any way you could be introducing a loop into the network?
It would help to know the exact model of all the equipment.