Ok it works. To summerize for others I use my system as example:
- Check ‘ip a’ :
root@OpenWrt:~# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master br-lan state UP qlen 1000
link/ether 00:e2:69:9e:33:5e brd ff:ff:ff:ff:ff:ff
3: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN qlen 1000
link/ether 00:e2:69:9e:33:5f brd ff:ff:ff:ff:ff:ff
4: eth2: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN qlen 1000
link/ether 00:e2:69:9e:33:60 brd ff:ff:ff:ff:ff:ff
5: eth3: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN qlen 1000
link/ether 00:e2:69:9e:33:61 brd ff:ff:ff:ff:ff:ff
6: eth4: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN qlen 1000
link/ether 00:e2:69:9e:33:62 brd ff:ff:ff:ff:ff:ff
7: eth5: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP qlen 1000
link/ether 00:e2:69:9e:33:63 brd ff:ff:ff:ff:ff:ff
inet6 fe80::2e2:69ff:fe9e:3363/64 scope link
valid_lft forever preferred_lft forever
8: br-lan: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP qlen 1000
link/ether 00:e2:69:9e:33:5e brd ff:ff:ff:ff:ff:ff
inet 192.168.1.1/24 brd 192.168.1.255 scope global br-lan
valid_lft forever preferred_lft forever
inet6 fda9:9e:825d::1/60 scope global noprefixroute
valid_lft forever preferred_lft forever
inet6 fe80::2e2:69ff:fe9e:335e/64 scope link
valid_lft forever preferred_lft forever
9: eth5.10@eth5: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP qlen 1000
link/ether 00:e2:69:9e:33:63 brd ff:ff:ff:ff:ff:ff
10: pppoe-wan: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1492 qdisc noqueue state UNKNOWN qlen 1024
link/ppp
inet 185.142.225.68 peer 109.233.104.27/32 scope global pppoe-wan
valid_lft forever preferred_lft forever
inet6 fe80::888a:cb2d:40f0:c33f/128 scope link flags 02
valid_lft forever preferred_lft forever
My WAN-IP (public IP) is 185.142.225.68
My WAN-Adapter is pppoe-wan
My LAN-IP (of the NTP-server) is 192.168.1.50
My LAN-adapter is br-lan
Check your own config makes it so much easier to do.
- nano /etc/nftables.d/90-stateless-ntp.nft and copy the code in it:
chain ntp_prerouting {
type filter hook prerouting priority raw;
iif pppoe-wan udp dport { 123 } ip daddr 185.142.225.68 ip daddr set 192.168.1.50 notrack
iif br-lan udp sport { 123 } ip saddr 192.168.1.50 notrack
}
chain ntp_postrouting {
type filter hook postrouting priority raw;
oif pppoe-wan udp sport { 123 } ip saddr 192.168.1.50 ip saddr set 185.142.225.68
}
alter it to your settings and reboot the router.
- check Status → Firewall it should show this at the top:
- Make a rule Network → Firewall → Traffic → Add:
- check the rule in ‘cat /etc/config/firewall’ it should read:
config rule
option src 'wan'
option dest 'lan'
option dest_port '123'
option proto 'udp'
option target 'ACCEPT'
option name 'NTP-notrack-with-extra-config'
list dest_ip '192.168.1.50'
option enabled '1'
- change the firewall connection parameters, the default is too low, ‘nano /etc/sysctl.conf’ mine looks like this:
net.netfilter.nf_conntrack_acct=1
net.netfilter.nf_conntrack_checksum=0
net.netfilter.nf_conntrack_udp_timeout=15
net.netfilter.nf_conntrack_udp_timeout_stream=180
net.netfilter.nf_flowtable_udp_timeout=15
net.netfilter.nf_conntrack_max=300000
activate with ‘sysctl -p’
- check the connections in the status window of OpenWRT, mine running with 3Gbit setting:
See traffic at >800 packages per second:
- Check if your incoming and outgoing still works. The Pool’s NTP-check is a good tool and should pass.
That’s it! Very major big thanks to @mlichvar who worked this out and explained it to me.
And sorry guy’s if I annoyed you, but I’m not a good firewall rule-writer.
I get there, in the end with some help. 
I hope this helps others that may have the same problem when traffic goes high.