Dual-stack IPv4&6 with Huawei E398 USB LTE stick (qmi)

Would you guys have any tips on troubleshooting IPv6 on LEDE 17.01.2 r3435 with USB LTE wan?

Here is my setup:
-Huawei E398 LTE stick

My ISP provides dual-stack IPv4 & IPv6, but the v4 address is NAT:ted. I have verified the v6 support by putting the SIM card into a Fritzbox LTE device, which get v6 address automatically.

Connection comes up nicely and I get a v4 address, but I have not been get the v6 up and running.

I have pasted below what I think are the relevant bits of the configs.

Any suggestions what to check and try would be most welcome.

config modem 'modem1'
option empty '0'
option uVid '12d1'
option uPid '1506'
option idV '12d1'
option idP '1506'
option device '1-1'
option baseport '0'
option maxport '3'
option proto '2'
option maxcontrol '/sys/devices/platform/ehci-platform/usb1/1-1/descriptors'
option manuf 'Huawei_Technologies'
option active '1'
option wdm '0'
option wwan '0'
option interface 'wwan0'
option commport '1'
option apn 'internet'
option user 'NIL'
option passw 'NIL'
option auth '0'
option model 'E398'
option celltype '3'
option modemtype '3'
option netmode '1'
option sms '1'
option smsnum '1'
option connected '1'
option cmode '1'


config globals 'globals'
option ula_prefix 'fdd4:f5bd:c676::/48'

config interface 'wan'
option ifname 'eth1'
option proto 'dhcp'
option metric '1'

config interface 'wan6'
option _orig_ifname 'eth1'
option _orig_bridge 'false'
option ifname 'wwan0'
option proto 'dhcpv6'
option reqaddress 'try'
option reqprefix 'auto'

config interface 'wan1'
option proto 'dhcp'
option ifname 'wwan0'
option _orig_bridge 'false'
option metric '10'