Eth 5 is the wan interface as shwn in the log as I unplugged it.You need to identify your actual WAN interface name and use that instead of eth0.
Apr 9 18:33:00 kernel: eth5 (Int switch port: 5) (Logical Port: 5) (phyId: 11) Link DOWN.
Apr 9 18:33:06 WAN_Connection: WAN(0) link down.
Apr 9 18:33:11 kernel: MACSEC: macsec is not enabled
Apr 9 18:33:12 kernel: eth5 (Int switch port: 5) (Logical Port: 5) (phyId: 11) Link Up at 2500 mbps full duplex
Apr 9 18:33:17 WAN_Connection: WAN(0) link up.
Apr 9 18:33:17 rc_service: wanduck 1547:notify_rc restart_wan_if 0
Apr 9 18:33:17 wsdd2[3715]: error: wsdd-mcast-v4: wsd_send_soap_msg: send
Apr 9 18:33:17 wsdd2[3715]: error: wsdd-mcast-v4: wsd_send_soap_msg: send
Apr 9 18:33:17 wsdd2[3715]: error: wsdd-mcast-v4: wsd_send_soap_msg: send
Apr 9 18:33:17 ddns: vlan101 has not yet obtained an WAN IPv6 address.(10)
Apr 9 18:33:17 ddns: WAN(0) IP is empty.(10)
Apr 9 18:33:17 kernel: ^[[0;33;41m[ERROR pktrunner] runnerUcast_inet6addr_event,148: Could not rdpa_system_ipv6_host_address_table_find ret=-5^[[0m
Apr 9 18:33:17 wsdd2[3715]: error: wsdd-mcast-v4: wsd_send_soap_msg: send
Apr 9 18:33:17 dnsmasq[3696]: interface br0 failed to join DHCPv6 multicast group: Address already in use
Apr 9 18:33:17 wsdd2[3715]: error: wsdd-mcast-v4: wsd_send_soap_msg: send
Apr 9 18:33:17 wsdd2[3715]: error: wsdd-mcast-v6: wsd_send_soap_msg: send
Apr 9 18:33:17 wan: mac clone: [wan0_hwaddr] == [D8:EC:5E:AC:AC:12]
Apr 9 18:33:17 rc_service: udhcpc_wan 12931:notify_rc start_rdisc6
Apr 9 18:33:17 wsdd2[3715]: error: wsdd-mcast-v4: wsd_send_soap_msg: send
Apr 9 18:33:17 wsdd2[3715]: error: wsdd-mcast-v6: wsd_send_soap_msg: send
Apr 9 18:33:18 rc_service: udhcpc_wan 12931:notify_rc start_dhcp6c
Apr 9 18:33:18 odhcp6c[12968]: Failed to send RS (Cannot assign requested address)
Apr 9 18:33:20 wan: finish adding multi routes
Apr 9 18:33:20 miniupnpd[2958]: shutting down MiniUPnPd
Apr 9 18:33:22 WAN_Connection: WAN was restored.
Apr 9 18:33:25 miniupnpd[13154]: HTTP listening on port 36853
Apr 9 18:33:25 miniupnpd[13154]: Listening for NAT-PMP/PCP traffic on port 5351
Apr 9 18:33:25 ddns: vlan101 has not yet obtained an WAN IPv6 address.(10)
Apr 9 18:33:25 ddns: update WWW.DNSOMATIC.COM default@dnsomatic.com, wan_unit 0
Apr 9 18:33:25 ddns: Clear ddns cache.
Apr 9 18:33:25 ddns: Start Inadyn(10).
Apr 9 18:33:25 inadyn[13160]: In-a-dyn version 2.10.0 -- Dynamic DNS update client.
Apr 9 18:33:25 inadyn[13160]: Update forced for alias all.dnsomatic.com, new IP# 80.79.49.38
Apr 9 18:33:25 inadyn[13160]: Updating cache for all.dnsomatic.com
Apr 9 18:33:29 BWDPI: fun bitmap = 43f
Apr 9 18:33:30 dhcp6_client: WAN Prefix Size Requested:/56, Received:/56
Apr 9 18:33:30 dhcp6_client: bound address 2a12:f43:1300::38f0/128, prefix 2a12:f43:1210:9600::/56
Apr 9 18:33:30 ddns: update WWW.DNSOMATIC.COM default@dnsomatic.com, wan_unit 0
Apr 9 18:33:30 ddns: Clear ddns cache.
Apr 9 18:33:30 ddns: Start Inadyn(10).
Apr 9 18:33:30 inadyn[13221]: In-a-dyn version 2.10.0 -- Dynamic DNS update client.
Apr 9 18:33:30 inadyn[13221]: Update forced for alias all.dnsomatic.com, new IP# 80.79.49.38<2a12:f43:1300::38f0>
Apr 9 18:33:31 rc_service: udhcpc_wan 12931:notify_rc stop_samba
Apr 9 18:33:31 wsdd2[3715]: Terminated received.
Apr 9 18:33:31 Samba_Server: smb daemon is stopped
Apr 9 18:33:31 inadyn[13221]: Updating cache for all.dnsomatic.com
Apr 9 18:33:32 rc_service: udhcpc_wan 12931:notify_rc start_samba
Apr 9 18:33:32 Samba_Server: daemon is started
Apr 9 18:33:33 dhcp_client: bound 80.79.49.38/255.255.255.0 via 80.79.49.1 for 600 seconds.
andy@RT-AX86U_Pro-4368:/tmp/home/root# tcpdump -v -n '(icmp6 and ip6[40] == 134) or (udp and port 547)' -i eth5
tcpdump: listening on eth5, link-type EN10MB (Ethernet), snapshot length 262144 bytes
^C
0 packets captured
0 packets received by filter
0 packets dropped by kernel
Still returns nothing