R750 phones won't register with SIP 403 during local survivability failover

Primary WAN cuts during the Tokyo ISP window and the R750 flips straight to local survivability on Edge 2024.6.1, but the phones won’t register. Edge logs are spamming SIP_REGISTRATION_FAILED: 403 Forbidden right after the switch. BIOS telemetry shows the NIC firmware’s stuck at 1.4.2 and doing jack all.