@hansome can there be a connection with this post?
Short summary so that you don't have to read everything. I had problems setting up Drop-In, but test now in a simple environment ISP Router - WAN MT-2500 LAN - client. PD/62 delegated. Whether OpenVPN or Wireguard is used, ping4 to google.com would work, ping6 not. Problem was the same in vanilla OpenWRT until I found out I have to do NAT66. Then I switched back to GL.Ient 4.6.2 where I found masq6 already set, but still ICMP6 request are routed outbound but no reply. I know IPv6 is not said to be supported, but I thought we can maybe find out why 
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BR,
Carsten.