Source: https://www.leowkahman.com/2016/03/19/disable-ipv6-raspberry-raspbian/
Firstly, check for presence of IPv6 using ifconfig. You should be seeing a few lines containing inet6 addr: ....
To disable, edit a file: sudo nano /etc/sysctl.conf
Add the following line:
net.ipv6.conf.all.disable_ipv6 = 1
For the change to take effect without rebooting: sudo sysctl -p
Verify that IPv6 address does not show up in ifconfig.
If you wish to re-enable, change the value in the setting above from 1 to 0 then execute the following two lines:
sudo sysctl -p
sudo ifconfig eth0 down && sudo ifconfig eth0 up
If you are doing this over SSH, the current session will of course be cut off. You should be able to reconnect in just a few seconds.
Fixed it with Network Manager on command line:
This command sets IPv6 method to "ignore", sets IPv6 privacy to "0" (optional, but it ensures IPv6 privacy extensions are disabled), and sets the connection to autoconnect. This ensures that the IPv6 configuration remains disabled even after a reboot.