Create the /jffs/scripts/nat-start file, and paste the following content:
#!/bin/sh
# don't route the local NTP servers (pihole @ 192.168.1.2) own outgoing ntp requests to itself, just accept them
iptables -t nat -A PREROUTING -p udp -s 192.168.1.2 --dport 123 -j ACCEPT
# route all other ntp requests (port 123) from the local network (192.168.1.0/24) to the local NTP server (pihole @ 192.168.1.2)
iptables -t nat -A PREROUTING -p udp -s 192.168.1.0/24 --dport 123 -j DNAT --to-destination 192.168.1.2:123