That is a weird one. One thing worth checking is whether the mobile provider is the same for all these users and if it has put your site or IP range on some kind of blacklist. It would explain why several people hit the same issue on mobile data but not on Wi-Fi.
A few other things that can sometimes cause this:
- mobile networks often default to IPv6, and if the server or proxy isn’t handling it properly, connections just fail.
- if you’re behind Cloudflare (or another CDN/firewall), it might be mistakenly blocking some mobile IP ranges.
- SSL/TLS quirks, some carriers still have issues with certain certificates if the chain isn’t perfect.
I’d try to gather from the users which providers they’re on, and whether they’re seeing the problem with IPv4 or IPv6. That should help narrow it down.