@Grav 

Hi, sorry for the delay.

Is your network structure like this:

ISP—main M9 plus(router mode)—switch—3 wired IP cameras

--if yes, what is the model number of the switch?

--have you set up the static IP address before?

--if unplug the switch, connect one wired camera to the main Deco directly, would camera get online?

Thank you very much and wait for your reply.