Lost IPv6 after upgrading to 18.06

Installing and Using OpenWrt Network and Wireless Configuration
21

I'm experiencing the same problem on TP-Link Archer C7 AC1750 V2 running 18.06.5

cat /etc/openwrt_release 
DISTRIB_ID='OpenWrt'
DISTRIB_RELEASE='18.06.5'
DISTRIB_REVISION='r7897-9d401013fc'
DISTRIB_TARGET='ar71xx/generic'
DISTRIB_ARCH='mips_24kc'
DISTRIB_DESCRIPTION='OpenWrt 18.06.5 r7897-9d401013fc'
DISTRIB_TAINTS=''

 cat /etc/openwrt_version 
r7897-9d401013fc
22

Did you try with the latest 19.07 rc2 stable ?

23

unfortunately, not. this is a production machine, and i am hesitating to put rc-s onto it...

24

Then you might try either 18.06.6 or the 19.07.0 release ...

25

I'm new to OpenWrt, and trying it on WZR-HP-AG300H.

19.07.2 seems to have a problem with IPv6, but there is a workaround. The following is my experience.

From Buffalo's ROM, I first flashed openwrt-19.07.2-ath79-generic-buffalo_wzr-hp-ag300h-squashfs-factory.bin, but I was never able to get the router be assigned an IPv6 address from my ISP that serves IPv6 by IPoE.

So I flashed lede-17.01.5-ar71xx-generic-wzr-hp-ag300h-squashfs-sysupgrade.bin, and IPv6 seemed fine. I got all green (except IPv6 Hostname) on https://ipv6-test.com/

Then flashed openwrt-18.06.8-ar71xx-generic-wzr-hp-ag300h-squashfs-sysupgrade.bin, and IPv6 still works.
I then flashed openwrt-19.07.2-ath79-generic-buffalo_wzr-hp-ag300h-squashfs-sysupgrade.bin, and IPv6 still works.