Hello,
I have the following setup:
- LHG LTE18 with a dual-stack APN with untagged management interface and tagged VLAN for LTE passthrough
- RB5009 with a bridge, untagged for managing the LTE device and other APs, tagged are LTE data and local VLANs
Since MT's pass-through does not support DHCP6-PD, the LAN VLAN uses an ULA prefix and an NPT firewall rule is installed. This worked fine in the past (although with hAP ac³ instead of RB5009) on RouterOS version 7.20.7. Upon powering on all systems the router received IPv4 address from DHCP and IPv6 from RA from the LTE device. The IPv6 address stayed permanently on the LTE Data VLAN and a custom script ran every minute to update the NPT firewall rule. This worked fine for months.
Last week I've replaced the hAP ac³ with RB5009 and upgraded RotuerOS to the next long-term release 7.21.5 . Since then the IPv6 address disappears from the LTE Data VLAN after a few hours. The default gateway stays installed over a link-local address. I am unsure of impact for clients' own traffic, since the NPT rule is still in place and the LTE device seems to be returning packets properly to RB5009, but without an IPv6 address on its WAN interface the RB5009 can't originate it's own IPv6 packets properly, that is for DNS and Wireguard tunnels I use for remotely managing the site. For the DNS issue the workaround was to upgrade to 7.23.3 because this version allows for /ipv6 nd settings set router-advertisement-ignored-options=dns so I can ignore the IPv6 DNS resolvers advertised by the mobile operator and the IPv4 ones will work fine.
Here's a fragment from logs:
# Rebooting the LHG LTE 18 device
2026-09-02 17:43:33 interface,info ether3 link down
2026-09-02 17:43:33 interface,info ether3 link down
2026-09-02 17:43:38 interface,info ether3 link up (speed 1G, full duplex)
2026-09-02 17:43:38 interface,info ether3 link up (speed 1G, full duplex)
# My script complains on no IPv6 address on WAN
2026-09-02 17:43:43 script,warning No IPv6 addresses found on vlan-LTE-Data
2026-09-02 17:43:43 script,warning No IPv6 addresses found on vlan-LTE-Data
# IPv4 does not work yet
2026-09-02 17:43:53 wireguard,info wg-nadir: [nadir-ipv6] xxxxx: Handshake for peer did not complete after 5 seconds, retrying (try 2)
2026-09-02 17:43:57 interface,info ether3 link down
2026-09-02 17:43:57 interface,info ether3 link down
2026-09-02 17:44:05 interface,info ether3 link up (speed 1G, full duplex)
2026-09-02 17:44:05 interface,info ether3 link up (speed 1G, full duplex)
2026-09-02 17:44:19 dhcp,critical,error VLAN-LTE-Data on vlan-LTE-Data lost IP address 10.157.76.251 - lease expired
2026-09-02 17:44:19 dhcp,critical,error VLAN-LTE-Data on vlan-LTE-Data lost IP address 10.157.76.251 - lease expired
# The LTE device finished booting, LTE passthrough works again, IP addresses are added on RB5009. Only IPv4 change is logged since it's DHCP, while the IPv6 change is from RA.
2026-09-02 17:44:21 dhcp,info VLAN-LTE-Data on vlan-LTE-Data got IP address 10.157.76.251
# After a few hours the IPv4 lease on WAN is renewed
2026-09-02 21:20:55 dhcp,critical,error VLAN-LTE-Data on vlan-LTE-Data lost IP address 10.157.76.251 - lease expired
2026-09-02 21:20:55 dhcp,critical,error VLAN-LTE-Data on vlan-LTE-Data lost IP address 10.157.76.251 - lease expired
2026-09-02 21:21:00 dhcp,info VLAN-LTE-Data on vlan-LTE-Data got IP address 10.157.76.251
2026-09-02 21:21:02 wireguard,info wg-nadir: [nadir-ipv6] xxxxxx: Handshake for peer did not complete after 5 seconds, retrying (try 2)
# Apparently the IPv6 address is gone from the WAN link because my script complains again.
2026-09-02 21:30:43 script,warning No IPv6 addresses found on vlan-LTE-Data
2026-09-02 21:31:43 script,warning No IPv6 addresses found on vlan-LTE-Data
IPv6 settings:
/ipv6 nd
set [ find default=yes ] hop-limit=64
/ipv6 nd settings
set router-advertisement-ignored-options=dns
/ipv6 settings
set accept-redirects=no accept-router-advertisements=yes accept-router-advertisements-on=WAN
The RouterOS version has been changed 7.20.7 -> 7.21.5 -> 7.23.3 both on router and the LTE device.