This instruction is tested valid for Debian 9.

Sign in as root mode.

Modify the sysctl.conf to enable IPV6 network.

Add the content as follows:

Save and exit.

Make the new settings be valid:

Modify the networking interfaces:

Note: Please check whether your network interface’s name is eth0, or change the name according to your vps.

For only one IPV6 address:

For more IPV6 addresses:

save and exit.

Restart your network:

Check the IPV6 settings:

After you set up the IPV6 successfully, the IPV6 gateway will prevail.
You could change the /etc/gai.conf to make the IPV4 gateway still be the default route.

Delete the # for this line:

Save and exit.

Thanks for reading.