DNS Issue (SERVFAIL / No Reachable Authority) – BIND + OVH

Environment
Dedicated server on OVH
OS: Linux
DNS server: BIND (named)
Nameservers:
ns1.academicogroup.com → 54.39.130.229
ns2.academicogroup.com → 15.235.37.59 (OVH failover IP)

Problem
Domains (e.g. academicogroup.com, formaccion.com.ec) are configured and working locally, but fail globally with:

SERVFAIL
No Reachable Authority

Example:
dig @8.8.8.8 academicogroup.com
→ SERVFAIL

What has been verified
BIND is running correctly
Zones are properly configured and loaded
SOA records respond correctly on both nameservers
Both NS respond locally
Glue records are correctly configured at the registrar
Nameservers are correctly delegated at the TLD level
Firewall (iptables) allows TCP/UDP port 53
DNS service is listening on both IPs
Failover IP is assigned to the server interface

Key Findings
From another OVH server:
dig @54.39.130.229 → timeout
dig @15.235.37.59 → timeout
Packet loss detected when pinging the failover IP
DNS resolution works only from some networks (mainly OVH)
Public resolvers like Google DNS return SERVFAIL

What I need
Looking for someone who can:
Diagnose the root cause of the DNS issue
Test DNS reachability from multiple external networks
Analyze OVH networking / failover IP behavior
Identify if the issue is network, routing, or DNS-related
Help restore stable global DNS resolution

Requirements
Experience with:

OVH infrastructure / networking
BIND DNS server
DNS debugging (SERVFAIL, delegation issues)
Failover IP configuration