The C4000XG has been sold to CenturyLink customers as a "modem" for their fiber tier service.
CenturyLink often sells it to customers for $20 or less, but it is available on Amazon for over $200.
I don't know who would buy a used C4000XG because while it works for some people (probably people who never touch it after the install tech leaves), it is full of software bugs. What kind of bugs you might ask? Well to start, it has a nasty habit of factory resetting itself once a month, though surprisingly the customer PPPoE credentials get preserved. Want to disable the router and use it as a plain old Wireless Access Point? Don't reboot it because DHCP gets re-enabled on every boot. Also there is a strange issue where some devices can't complete their DHCP handshake with the real router...
Ya ok... well if it so bad, why write this blog? Because the AX (WiFi-6) wireless radio is amazing. At least with a small number of devices, it seems to beat out a WiFi-5 Ubiquiti access point for cutting through the noise. I suspect due to the antenna layout. It also interestingly has an SFP+ port, possibly CenturyLink hoped to replace the ONT?
So ladies and gentlemen, I present to you a sneak peek of the inside.
The cutout in the heatsink above the capacitor appears to be a serial connection with header pins labeled: TX, GND, VCC, RX
Is also held in with 6 retention clips and hides 2 screws holding the circuit board in place. Three can be easily found under the heatsink on one side, but the other three... only one can be accessed.
At this point I was done for the night, till next time when hopefully I have more information about what is inside...