Сделано на основе NetworkManager + dhcpcd5 + radvd

• Внешний интерфейс enp6s0, ppp0
• Локальная сеть enp0s25

/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

/etc/NetworkManager/system-connections/DomRuPPPoE.nmconnection

[connection]
id=DomRuPPPoE
uuid=478696dd-40e6-445c-8a15-09f1af0f1057
type=pppoe
interface-name=enp6s0
permissions=
timestamp=1661360954

[ethernet]
mac-address-blacklist=

[ppp]
lcp-echo-failure=5
lcp-echo-interval=30

[pppoe]
password=mypassword
username=v2076796

[ipv4]
dns-search=
method=auto

[ipv6]
addr-gen-mode=stable-privacy
dns-search=
method=auto

/etc/dhcpcd.conf

allowinterfaces ppp0 enp0s25
duid
ipv6only
noipv6rs
interface ppp0
    static ip6_address=2a03:1ac0:5e4f:29df::1111/64
    ia_pd 1/::/64 enp0s25/0/64

/etc/radvd.conf

interface enp0s25
{
  AdvSendAdvert on;
  MaxRtrAdvInterval 60;

  prefix ::/64
  {
    AdvValidLifetime 600;
    AdvPreferredLifetime 120;
  };
};

systemctl enable NetworkManager NetworkManader.dispatcher radvd dhcpcd --now