My web server is (include version): Mutiple tested.
My hosting provider, if applicable, is: Xfinity
Using Certify The Web on Windows I keep getting a timeout error form Lets Encrypt. However I have had no problems loading the challenge url from multiple other ISP's.
That looks like it is probably the issue. I only tested from US addresses. I will have to look at the upstream routers and see if I can find any Geo rules.