Hello!
I got myself a Gl.inet AX1800 (Flint) router and a Zyxel VMG3006-D70A modem for my home network, dialing into my ISP via PPPoE. I am able to load 'established websites' like google.com, instagram.com on WiFi and cable connected client devices but only few other sites.
I spent two weeks trying to figure this out by myself but I've to admit that I'm out of my depths here. Last time set up a network manually was a 56kbps modem... 
I intend to set up a basic home network:
Modem <-> Router <-> Client devices (desktop, laptop, smartphones)
I hope I can present this case in a coherent fashion with all relevant details right away.
I appreciate any help as this connection needs to be operational rather soon.
Thank you!
ISP is WinSIM / Drillisch GmbH, recognized by Speedtests as 1&1 Versatel Deutschland GmbH (Guten Tag!)
-- Their settings are:
---- LAN tag 7 [check, set in modem]
---- Native IPv6 [check, set in router]
---- IPv4 pver DS-Lite [I could not find info on how to set this!]
---- I see in their documentation that other Fritz Boxes have a p-bit set to 0, my modem mentions it and shows the Hex table but I don't know if and how I set the p-bit. Is it relevant at all?
Modem is Zyxel VMG3006-D70A
-- Modem is not branded, not from a provider, latest firmware installed from 2022
-- VLAN tag set to 7 for the WAN facing LAN port
-- Default IP tables, 192.168.100.1, Subnet 255.255.255.0
Router is GL.inet AX1800
-- 4.5.16 stable firmware
-- no VLAN tag is set in the PPPoE connection, it dials in
-- IPv6 activated, set to Native as requested by ISP, DNS acquisition method: Automatic
-- MAC address factory default
-- Router IP default: 192.168.8.1 | Netmask default: 255.255.255.0
-- Router DHCP Server enabled, from 192.168.8.50 to 192.168.8.249
-- If I set ethernet Status Track on 'both ipv4 & ipv6' or 'ipv6', the router becomes inaccessible from my desktop and can't ssh into it either through its 192.168.8.1 IP
-- DNS -> override DNS Settings for All Clients can both be Encrypted DNS or Automatic (DNS from Ethernet (IPv6): 2001:1438:2:4::8,2001:1438:2:3::8), yields the same result (=see below, what works)
What works:
-- The router is calling successfully with its PPPoe credentials into the ISP
-- I can open well established websites like google.com, instagram.com (web interface and the app via wifi connected devices) with WiFi and cable connected devices
-- I can run a (Vodafone) speedtest: 272Mbit/s down, 44Mbit, 8ms ping, up which is expected
-- Diagnostic results from Luci's network utilities:
---- IPv4 Ping: PING openwrt.org (64.226.122.113): 56 data bytes | ping: sendto: Network unreachable
---- IPv6 Ping: 5 packets transmitted, 5 packets received, 0% packet loss | round-trip min/avg/max = 7.922/8.328/9.211 ms
---- IPv4 traceroutes: traceroute: can't connect to remote host (64.226.122.113): Network unreachable
---- IPv6 traceroutes: traceroute to openwrt.org (2a03:b0c0:3:d0::1a51:c001), 30 hops max, 64 byte packets
---- NS lookup: Server: 127.0.0.1 | Address: 127.0.0.1#53 | Name: openwrt.org | Address 1: 64.226.122.113 | Address 2: 2a03:b0c0:3:d0::1a51:c001
Here are my blind spot and the things I have not changed:
-- I have not set firewall rules
-- I have not manually entered DNS
-- No Drop-in Gateway
-- I have no set any: port forwards, open ports on router,DMZ
-- No address reservation
-- No static IPs set for client devices
-- I avoided the 'The interface is connected but the internet can't be accessed.' by flipping Multi-WAN -> Ethernet -> to: 'Internet always available when the interface is connected'
-- I see in the logs:
Wed Jul 3 12:19:24 2024 daemon.warn dnsmasq-dhcp[9208]: DHCP packet received on eth0 which has no address
...and I sense that it is important but here is where I can't progress further by myself.
I appreciate any help in this matter! 
I'm happy to see you posting this thread, I saw it myself just yesterday. It means I was on the right track, I just couldn't puzzle it together. I give it a read now
