Hyperlinks work in Preview/Test email, not in actual sent email. ISP issue?

I had a colleague test the same at her location, but she got the same result as she has the same ISP.

I also tested out the hyperlinks on my mobile device (iPhone) using cellular data only and all links opened correctly in Safari. When I tested my mobile device while connected to Xfinity wi-fi, the hyperlinks do not open.

This issue I believe started within the last month, given that the last email we sent on Jan 17 and initially worked (but now doesn't).

Emails are being delivered according to the reporting. The issue is not limited to one particular email. How can this be resolved with Xfinity (large ISP) by Constant Contact?