webserver issues

After going round and round with my ISP for a couple of weeks, we have determined that Comcast has some issues resolving web pages. For example:

https://www.newsletterbroadcast.net/login

This page will not resolve on Comcast, but works fine on Verizon.  Need to open a ticket with Comcast, but there is no way to actually communicate with Comcast if you are a retail customer.  Help!