Сделано на основе NetworkManager + dhcpcd5 + radvd

/etc/NetworkManager/NetworkManager.conf

[main]
plugins=ifupdown,keyfile

[ifupdown]
managed=false

[keyfile]
# отключение USB модема, использовать для asterisk
unmanaged-devices=type:gsm

[logging]
#level=WARN
#domains=DHCP6:INFO,PPP:INFO,IP6:INFO

/etc/NetworkManager/dispatcher.d/10-defaultroute6feser

#!/bin/bash

IFACE=$1
ACTION=$2

if [ -n "$IFACE" -a "$IFACE" = "ppp0" ]; then
    if [ -n "$ACTION" -a "$ACTION" = "up" ]; then

        ip -6 route add default dev $IFACE
        systemctl restart dhcpcd.service

    fi

    if [ -n "$ACTION" -a "$ACTION" = "down" ]; then

        ip -6 route del default dev $IFACE

    fi
fi