Enable packet forwarding via interfaces:

# cat /etc/sysctl.d/01-forward.conf  

net.ipv4.ip_forward = 1
net.ipv6.conf.all.forwarding = 1
net.ipv6.conf.default.forwarding = 1

Then install isc-dhcp-server and configure ipv4 and ipv6 dhcp server. (only on local ports or you internet prowider will be angry)

short example:

# cat /etc/dhcpd.conf  
ddns-update-style interim;
ddns-updates on;
ddns-domainname "my.local";
ddns-rev-domainname "in-addr.arpa";
allow client-updates;
update-conflict-detection true;
update-optimization true;
authoritative;
default-lease-time 86400;
preferred-lifetime 80000;
max-lease-time 86400;
allow leasequery;
option domain-name "my.local";
option domain-name-servers 192.168.1.1;
lease-file-name "/var/lib/dhcp/dhcpd.leases";

# cat /etc/dhcpd6.conf  
ddns-update-style interim;
ddns-updates on;
ddns-domainname "my.local";
ddns-rev-domainname "ip6.arpa";
allow client-updates;
update-conflict-detection true;
update-optimization true;
authoritative;
default-lease-time 86400;
preferred-lifetime 80000;
max-lease-time 86400;
allow leasequery;
option domain-name "my.local";
option dhcp6.name-servers fd00:1::1;
option dhcp6.domain-search "my.local";
option dhcp6.preference 255;
dhcpv6-lease-file-name "/var/lib/dhcp/dhcpd6.leases";

don’t forget start dhcpd@lan and dhcpd6@lan

Then install radvd and configure RA ipv6 broadcasting. (only on local ports or you internet prowider will be angry)

# cat /etc/radvd.conf

interface br0
{
        AdvSendAdvert on;
        MinRtrAdvInterval 3;
        MaxRtrAdvInterval 10;
        AdvDefaultPreference low;
        AdvHomeAgentFlag off;

        prefix fd00:1::/64
        {
                AdvOnLink on;
                AdvAutonomous on;
                AdvRouterAddr off;
        };

        RDNSS fd00:1::1
        {
                AdvRDNSSLifetime 30;
        };

        DNSSL my.local
        {
                AdvDNSSLLifetime 30;
        };

};

Then install iptables-persistent and configure ipv4 and ipv6 rules in /etc/iptables/ . Change lan and internet to you real interfaces.

# cat /etc/iptables/rules.v4
# Generated by iptables-save v1.6.1 on Mon Dec 30 18:53:43 2019
*nat
:PREROUTING ACCEPT [0:0]
:INPUT ACCEPT [0:0]
:OUTPUT ACCEPT [0:0] 
:POSTROUTING ACCEPT [0:0]
-A POSTROUTING -o internet -j MASQUERADE
COMMIT
# Completed on Mon Dec 30 18:53:43 2019
*filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [0:0]
#UNBRICK IF YOU WANT ACCESS FROM INTERNET
-A INPUT -s x.x.x.x -j ACCEPT
-A INPUT -s y.y.y.y -j ACCEPT
#BASE
-A INPUT -i lo -j ACCEPT
-A INPUT -i lan -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i lan -j ACCEPT
-A FORWARD -p icmp -j ACCEPT
-A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
COMMIT

# cat /etc/iptables/rules.v6
# Generated by ip6tables-save v1.6.0 on Thu Sep  8 13:29:11 2016
*nat
:PREROUTING ACCEPT [0:0]
:INPUT ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
-A POSTROUTING -o internet -j MASQUERADE
COMMIT

*filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [0:0]
#BASE INPUT
-A INPUT -i lo -j ACCEPT
-A INPUT -i lan -j ACCEPT
-A INPUT -p ipv6-icmp -j ACCEPT
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i lan -j ACCEPT
-A FORWARD -p ipv6-icmp -j ACCEPT
-A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
COMMIT

Then install dns relay. I user bind, but that some overkill. But anyway:

install named / bind9

# cat /etc/named.conf

...
acl "lan" {
           192.168.1.0/24;
           127.0.0.1;
           fd00:1::/64;
           ::1/128;
};

tls google-DoT {
    ca-file "/var/named/google.crt"; //SET google cert path here
    remote-hostname "dns.google";
};

tls local-cert { //if you want local SSL requests
    cert-file "/etc/letsencrypt/live/local/cert.pem";
    key-file "/etc/letsencrypt/live/local/privkey.pem";
};


options {
    directory "/var/named";
    pid-file "/run/named/named.pid";

    forwarders port 853 tls google-DoT {
      8.8.8.8;
      8.8.4.4;
    };

    // Uncomment these to enable IPv6 connections support
    // IPv4 will still work:
    //listen-on-v6 { any; };
    // Add this for no IPv4:
    //listen-on { any; };

    listen-on-v6 { fd00:1::1; ::1; };
    listen-on { 192.168.1.1; 127.0.0.1; };

    listen-on-v6 tls local-cert { fd00:1::1; ::1; }; //if you want local SSL requests
    listen-on    tls local-cert { 192.168.1.1; 127.0.0.1; }; //if you want local SSL requests

    allow-recursion { lan; };
    allow-recursion-on { 192.168.1.1; fd00:1::1; 127.0.0.1; ::1; };
    allow-transfer { none; };
    allow-update { none; };
    allow-query { lan; };
    allow-query-cache { lan; };
    allow-query-cache-on { 192.168.1.1; fd00:1::1; 127.0.0.1; ::1; };

    version "DNS Server 1";
    hostname "interesting server";
    server-id "realy interesting server";

    dnssec-validation auto;
    empty-zones-enable no;
    minimal-responses yes;
    http-port 8888;

    listen-on http local tls none { any; };
    listen-on-v6 http local tls none { any; };

    auth-nxdomain no;    # conform to RFC1035
};
...

All done.