So I have managed to make progress, and I’m just updating hoping that others might benefit if they are a noob on Linux like I am.
So the error above wasn’t the full error, I had to run the command “journalctl -xe” and then I saw that it was not finding subnets set up. This comes in the next step. So after rebooting again, I then proceeded to set up the subnets and ranges, and then the error seemed to go away. At this point I then went to the link provided by Zane [link to wiki.debian.org] to set up the bridge between the lan & wan connectors. I’m not quite sure yet how much on that page relates to what I need, but I’m working through it. What I did find, is that when I connected my laptop to the lan port, it got the IP .100 so that is good news, although at this point it didn’t yet have internet access. Progress

Thanks for responding.
In the process of setting up the RPi as a router, I get to the following 2 commands:
sudo systemctl disable dhcpcd
sudo apt install isc-dhcp-server
The first runs fine, but the install command brings up an error right after the line where it says generating the /etc/default/isc-dhcp-server… (hope I type it out correctly)
Job for isc-dhcp-server.service failed because the control process exited with error code.
See “systemctl status isc-dhcp-server.service” and journalctl -xe” for details
invoke-rc.d: initscript isc-dhcp-server, action “start” failed.
isc-dhcp-server.service – LSB: DHCP server

And then further down it has one of the lines as:
….date and time…raspberrypi isc-dhcp-server[1076]: Starting ISC DHCPv4 server: dhcpdccheck syslog for diagnostics…failed!

So has it failed because dhcpdc has been removed and so no syslog exists?

Yes, using the RPi as a router (by definition) means that it is the DHCP server assigning the IP addresses to the devices in the network. Yes, you must bridge the traffic between the two eth interfaces on the RPi such that the internet connection (WAN) becomes available to the LAN. If that’s where you’re stuck, the Debian Wiki has good information about bridging network connections.

My intended setup is this – I plug my RPi directly into the ONT so that I can monitor all traffic. Because I also want to monitor different devices connected to the network, the RPi will act as the DHCP server (correct?). So my first link to follow will be “Building your own router with a Raspberry Pi” using the RPi OS Lite.
The existing WiFi router that I have will be set to pass-through (bridge) mode. Is this so that devices can still connect wirelessly to the network, but their assigned IP’s will be assigned from the RPi which is connected to the WiFi router’s WAN port?
Then in your other article, the network monitoring one, you talk about setting up the eth1 eth0 connection. I don’t understand the syntax. Do you mean setting up allowing traffic from outside (ONT) connected to eth1 to flow through the RPi to be monitored and then out eth0?
Just to mention, I have got to the point where I managed to run the pppoeconf and it connected to the internet.
I have a few more questions but I will hold off until I get this part correct.

As a matter of practice, I’d start with rebooting everything. Is the switch managed or unmanaged? Does the ping drop happen at a specific point in the traceroute, e.g., the router? If it’s definitely happening at the Pi, how many devices are on your network? Are you using an older Pi? Are you running other software simultaneously on the Pi? What does the CPU/memory load look like? Can you measure the throughput to see how congested the network is? Somewhere, some resource on the Pi must be tapped out if it’s creating such a bottleneck.

Actually, I keep fixing things and then finding other issues. My kids started moaning about the xbox and a ping (directly connected to the switch) returns MANY dropped pings. Speedtest however seems fine. Rebooted everything but still getting loads of packet loss. Ifconfig doesn’t show any errors or collisions.

Thanks, I’ve actually got around this by using your info on the DNS re-write with AdGuard 🙂

Another issue I appear to have is that AdGuard doesn’t appear to be blocking anything. It says it is in the console and “looks” to be working however I can do some Ad block tests and none work. There must be some form of DNS leakage. If I stop the service nothing resolves so that part is working. If I do a DNS check I get some random 141 address which is apparently Apple Private relay (which I’m not using and isn’t released yet!) I have the upstream set as CloudFlare family but that too doesn’t block any adult content so clearly isn’t working either. At a bit of a loss. All clients are getting the correct router address as their DNS server.

The way I do this is to use a locally hosted DNS server. You could use PiHole, a DNS server on a Synology NAS, or one of many other options. Just manually add entries for each FQDN to the DNS server with the local IP; there’s no need for more sophisticated NAT loopback techniques. As a bonus, a locally hosted DNS serves as a cache which can help lookup times in certain situations.

Only item I’d stuck on is the NAT loopback. I have some sites hosted on a NAS that I can get to externally using the URL however can’t internally. It would be great if I could get to these internally using the DNS rather than IP.