Cannot connect to one specific web based email site

So I have the wierdest issue that came up a month ago and was ony solved by getting a new modem from Comcast...

My wife is unable to get to the login page of her work email website while on our network at home... It had been working for the last month, but now it doesn't... In the middle of the day it just stopped.

She can get to the website and log in if she goes on her phone and turns off wifi, although if wifi is on it wont work..

If we use a neighbor's wifi (with permission of course), she can log in...

We have tried everything Comcast tech support has brought up and nothing has helped... 

is it possible the my modem is getting blacklisted?