Ask HN: Interesting Technical issue (Internet/Network) that baffles me

So my Father-In-Law calls me first for any "tech" issues with his laptop etc and I take pride in that. I am not an expert in networking and a really interesting issue has come up. He visits a banking website and when he enters the username/passwd on the login screen, he gets some type of a 500 error. I checked this on his laptop and replicated the issue. I checked the Chrome inspector and it does show hitting the servers of the bank so the connection is def. not dropping plus the error clearly is from their server. Here is the "interesting" part about it. It ONLY happens when he tries to do it on HIS home network (verizon). If he tries any other network using the same laptop, this issue does not happen. Ok so think we have narrowed the issue down to HIS network. Fine. I ask him to clear his browser cache, clear the DNS cache, reboot his Router and reboot his laptop. Still no change. He calls verizon and spends hours with them on phone. He goes to the bank's local office and they cannot tell him anything. Would any network experts like to suggest something here ? This is not end of the world as he can clearly do this outside his home network but this is annoying and for me, it is curious.