Conor

ISP throttle on the traffic so if your CCTV sytems aren't sending any info while connected then there would be no effect on the POS. It may be a recent upgrade of the firmware on the comcast router that say defaulted a LAN port to a WAN port. So i'd check the ports on the comcast in its config. I would also use a simple 5 port dumb switch to check each port. Now there is one other thing i have seen in the past and that was where the ISP would only permit a single MAC address connected to its ISP modem, this was to stop internet sharing. So you can only have a single firewall connected etc. Comcast seem a bit big to do that though. You can test that with a single cheap tplink router between the ISP and the cameras and firewall etc.

the only other obvious thing is to make sure that the NVRs and firewall are all on a private static ips outside the comcast dhcp pool, if one is static and another a dhcp, then you could have then overlapping and interfering with each other.

This sounds like your cameras are either hacked or being streamed 24/7, that would max out your uplink and hence the POS is struggling. I would run a tcpdump on the traffic from those DVRs to work out if its a hack or your owner is streaming from them. In future access your DVRs not via the comcast ip or cloud but via a VPN, if you aren't used to VPNs then use an easy to deploy vpn via an i-spi from netcelero. (www.netcelero.com)

Web sites are basically just folders of files, what it is showing you is a folder with what looks like no files. But without a make and model its hard to know what you should be seeing. Also you would need to confirm how you know its that ip address and you aren't connecting to something else in the house.

I'm curious, why are you using a TP wifi extender, why not just use a TPlink powerline adpater for connecting to the ethernet port on the DVR in the attic and the ethernet port on the back of your home ISP router downstairs?

Just to add for others whom may stumble across this post. 4G/5G Mobile/Cellular operators use one of three methods to provide you with an Public IP address. 1. Static IP on your router (you can set up port forwards with this type) 2. Dynamic Public IP on your router (you need dynamic DNS or some other method to know the current IP, but port forwards can be gotten to work) 3. Carrier NAT (CGNAT) your public IP sits in the carriers core network, you can't use port forwards. I have uploaded an image below to illustrate this. For these you need VPNs etc. (Sorry Blatant sales pitch: check out Netcelero.com) Also at time of posting STARLINK is CGNAT.