Slow Wired Internet Speed Compared to Modem

I have Spectrum Internet with 50 Mpbs down and 5 Mbps Up. Recently I noticed my internet download speed is much lower than it should be (~ 3 Mbps). I worked with a Spectrum agent and I could get 50 Mbps down if my Netgear CM600 Modem was directly connected to my computer, but it only gets 3 Mpbs down when wired through the router (Nighthawk AC1900). I installed the latest firmware via manual download and factory reset the router (V1.0.8.34_1.2.15). This did not fix the issue. I replaced cables and cycled both the modem and router several times to no avail. I even tried rolling back the firmware with older versions to no avail. Everything use to work perfectly 50 Mbps down and 5 up through the router until recently. I haven't made any recent changes. I have QoS disabled. WMM is enabled (both options).  Spectrum told me I have an IPV6 address. I suspect the problem may be on their end, since the router is still technically working. I'm perplexed here folks. Any help would be much appreciated.

Thanks.