I'm in a battle between GoDaddy and ATT.Net about access to my web site. If you use ATT.Net as your ISP please go to lignumturner.com and let me know if the page opens for you. It's just a small vase I stuck in there as a sample. One or two from other ISPs might be helpful as well.
Works fine going through Verizon Jet Pack (cellular).
My best guess, it is AT&T. I have encountered it a couple times in the past with a web site I used to maintain for our church. It is not easy to get them to fix it. Likely cause is that previously some jerk was spamming from an IP number belonging to GoDaddy and AT&T blocked it (maybe even a series of IP numbers). You are now being assigned that number. I'd talk to both GoDaddy and AT&T and try to get them to work together. Won't be easy.