LinuxfarmerHH:

Supply the following if possible:

• Device problem occurs on
  TP-Link Archer C7 2.0
• Software versions of OpenWrt/LEDE release, packages, etc.
  Sysupgrade 18.06.2
• Steps to reproduce

Upgrade 18.06.1 with ds-lite module on a ds-lite connect with 18.06.2

Result: Your router is offline after reboot because the old ds-lite module is gone. Without the module you don't get a IPv6 prefix and so you also don't get the WAN6_4 to provide a IPv4 connect.

This is a killshot and dead end, the module must be fiddled offline into the router with an usb key.