Hold on a minute. There shouldn't be more than one "modem." The modem is basically just the bridge between your network and the cable company's. There should be one device that will establish a single connection to their network. Their DHCP server will grant the modem a single IP address that will appear as the WAN address in your router. The router routes requests from devices on your network to their modem by granting IP addresses that only work on your network (typically in the 192.168.x.x range).
Because of the way the cable networks are designed it is theoretically possible to have 2 modems but I'd be surprised if their network would dole out 2 IP addresses to the same physical address.
I'm confused about all of the devices you have. Assuming Spectrum is the service provider I suppose it is possible for you to have 2 modem-like devices - one for your telephone and another for Internet access but usually that's all in one device. If their device is acting as a router, and you've introduced another router then that's going to cause problems.
I don't know what brand of camera system you have, but *usually* the wireless cameras that come in the camera/DVR kits do not require internet access - they talk to the DVR wirelessly. The DVR is connected to the Internet and provides the remote viewing capabilities. If this is the case, those cameras are not on your WiFi network and aren't consuming any bandwidth. If they are accessing the Internet I'd have to wonder why - there would be no need for the DVR.
If you're consistently testing at the promised-speeds while hardwired to the modem then I'm suspecting IP address conflicts/collisions due to more than one device acting as a router.
As I asked before: Did it ever work right? If so, and it doesn't now, what has changed?