Senior Network Security Engineer – Diagnose ISP-Specific SSL/TLS Website Issue

We are looking for a senior network/security engineer to diagnose a complex website connectivity issue affecting a subset of users. This is NOT a WordPress issue. The website itself functions normally for most visitors. The problem appears to affect only certain ISP/network paths. Symptoms Website works normally for most users. Some users receive: ERR_SSL_PROTOCOL_ERROR "Unable to establish a secure connection" Safari SSL errors One confirmed user on T-Mobile 5G Home Internet cannot access the site but can access it immediately when switching the same phone to cellular data. A previous user regained access after contacting their ISP. Google Safe Browsing has already cleared the domain following a previous security incident. Cloudflare is being used in front of the origin server (Pressable). We need someone to identify the actual root cause—not simply confirm the site works. Possible areas include: ISP filtering/security products Network routing IPv4 vs IPv6 TLS handshake failures Cloudflare edge behavior DNS behavior Hosting/origin connectivity Security reputation systems CDN configuration MTU/path issues WAF/security rules Deliverables Please determine: Where the failure occurs Before Cloudflare At Cloudflare Between Cloudflare and origin At the origin server Root cause Evidence supporting the diagnosis Recommended permanent fix Any changes required to Cloudflare, Pressable, DNS, SSL, or network configuration Required experience Advanced TLS/SSL troubleshooting Cloudflare CDN architecture IPv4/IPv6 networking DNS Network routing ISP troubleshooting Packet capture/Wireshark Security reputation systems HTTP/HTTPS diagnostics Please include examples of similar issues you've solved. This is not a WordPress development project.