Can you go to Settings page and check under Advanced, NTP server, what’s the NTP server IP there? The latest firmware has a logic where if NTP IP is 0.0.0.0 it uses the default ntp.pool.org, otherwise it uses your specific NTP IP. At factory reset, it set NTP IP to 0.0.0.0 so it uses the default ntp.pool.org. However, if you upgraded from a previous version of the firmware, it may have some other NTP IP there, which may have stopped working reliably.