ipv6 – Why might someone be denied access to my website from a mobile hotspot?

Full disclosure: I asked this question on “Network Engineering” and it was quickly closed as off-topic and the moderator sent me here.

Recently, I started self-hosting my academic webpage with a VPS. One of my students informed me that he could not load the webpage. We figured out that the problem was that he was connecting to the internet with a mobile hotspot. He switched his connection to his roomate’s wifi and could connect to my page with no issues.

In short, my question is: Why might this happen?

My guess is that I didn’t set-up ipv6 properly. My server runs arch linux. During the os install, I followed the installation guide and my /etc/hosts file is:   localhost
::1     localhost   myhostname.localdomain  myhostname

Here, I replaced with my server’s static ipv4 address. Should the ipv6 address be accounted for here? If so, how?

Finally, this issue has made me realize that I do not have access to a device with an ipv6 address so I’m not sure how to test this issue myself. So, my followup question is: How can I test if my ipv6 connection is working properly without a device with an ipv6 address?