R6900 Seems to block certain web sites for no reason

Nighthawk R6900 Firmware Version V1.0.1.22_1.0.18 (Latest Version) Windows 10 Home Version 1703 Samsung Galaxy S8 Recently I had problem accessing Craigslist.org and then later iomoio.com . At first I thought my ISP, which is Spectrum was blocking these domains. As part of my troubleshooting I tried using different browsers and different devices. All of these urls gave me timeout errors. I could access these urls if I used my cell phone without Wi-Fi access or if I used it as a hotspot for my laptop. away from home. So I used a proxy server through my desktop and I was able to access the urls. Next I connected my laptop directly to my cable modem and was able to access the urls. At this point it seems that my router is the problem. After accessing the administrative functions of the router I could not find any reason why my router would block these urls. I have no parental restrictions in place, no blocked ip addresses specified. I cannot find any apparent cause for these urls to be blocked. So I'm out of ideas at this point and am looking for help. Thanks.