Simple router without NAT on Raspi

Dear OpenWRT community,

this would be a very nooby question, please don't be too harsh.

I have a slow ADSL internet connection, my trusty AVM modem/router does not mange to priorize traffic to my (home)office pc in a sophisticated way. Other PCs watching Youtube clog up the internet.

What I wanted to achieve:
Using a Raspi as a "gatekeeper" to priorize network traffic to my office machine.

What I got:
Configure ETH0 to a static IP, assign LAN zone, resolve and ping hostnames in the internet.
Client successfully could access the pi on its ETH0 IP.
The drivers for the USB NIC were also successfuly installed.

ETH0
IP static 192.168.1.5
Subnet 255.255.255.0
Gateway 192.168.1.2
DNS 192.168.1.2
DHCP off
zone LAN

ETH1
IP static 192.168.1.6
Subnet 255.255.255.0
Gateway 192.168.1.5
DHCP on
zone LAN

Now I am stuck.
Connecting a client to ETH1, it obtains a DHCP IP (but not in the range specified) and even refuses to ping 192.168.1.6. How can that be?

Could you please help me get unstuck?!
Untitled Diagram

Thanks in advance and regards

What IP does the client get?

DHCP is set to start 100.

The client gets
192.168.1.178
255.255.255.0
broadcast 192.168.1.255

178 is higher than 100?

IPs aren't assigned in any special order.

Indeed it is, did not know, IPs are assigned in a various order.
Activating Bridge Interface made ping from the client on ETH1 possible to 192.168.1.6 and 192.168.1.5, but not 192.168.1.2 .

A client on ETH0 is no longer able to ping or access 192.168.1.5 or 192.168.1.6, nor the client connected to ETH1.

Would a static route be needed, and if: How shall it look like in this case?

Merely setting up SQM on the whole WAN usually makes ADSL work dramatically better even without putting clients into different classes.

The conventional way to do this with a router is to route between networks with different IP subnet ranges. Within the same subnet, there really isn't a concept of routing.

Though you could probably just bridge the two ports and apply SQM to the one connected to the modem, that isn't the most expandable way to approach this problem.

The dnsmasq DHCP server used in OpenWrt assigns IPs within the allowed range by hashing the client's MAC address. This makes the IP's appear random, but it also means that a client with a given MAC is more likely to always get the same IP.

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