I have some weird issues with IOT's and NTP, also have some devices that I don't want talking to the Internet so I have OpenWRT set up as an NTP Server.

Before moving forward, make sure your OpenWRT router is an NTP Server and not just an NTP Client, for this, log into your OpenWRT Console, go to System, then System again, and open Time Synchronization and check the Provide NTP Server.

Now go to Network, then Firewall, Custom Rules and add the lines below, the address 192.168.1.0 is my network IP range, and 192.168.1.1 is my OpenWRT IP, which is my NTP server.

If you have more than one IP range, add the same line with their individual IP range and interface IP's.

# Force NTP

iptables -t nat -A PREROUTING -s 192.168.1.0/24 -p udp --dport 123 -j DNAT --to 192.168.1.1

iptables -t nat -A PREROUTING -s 192.168.1.0/24 -p tcp --dport 123 -j DNAT --to 192.168.1.1