Hi everyone

I am trying to get a D-Link Router DIR-615 H1 with OpenWrt working. As I tried several things but couldn't get it work, I ask for help here. Thank you in advance for your support!

Trial set-up
(Background for the trial: see below)
Goal: connect two subnets with a D-Link DIR 615 H1 let's call this router D.
Router E is attached to the internet. Router F (an old ZyXel) creates a second subnet. Router E and F are not connected at this stage.
Now the D-Link is attached between the two. On both routers (E and F) I added a static route for the other subnet with the IP of the D-LInk in the corresponding subnet as next hop.
I first tried with the stock firmware of the D-LInk, unsing the wan port for subnet F. Couldn't get it run.
Then I tried dd-wrt. Switched mode to routing. No success either. As dd-wrt is not supporting vlans on non-Broadcom devices and the GUI is difficult to understand for a newbie, I decided to try OpenWrt.
Here I tried four things
1) Reconfigure the firewall settings for the wan interface and add an additonal port to the vlan. Then connecting the F subnet to the one of the tow ports.The E subnet to one of the remaining lan ports. No success.
2) Add a new interface with one port for the F subnet. Add the interface to the same zone as "lan" in the firewall
3) Same as 2) but a new zone in the firewall
4) Same as 2) but disabled firewall

Interesting: tracert (with an IP of the E subnet) in the F subnet arrives at the D-Link router, but does not go further. Unless it is the E-IP of the D-Link. That works. Tracert in the E subnet (for an F subnet IP) is sent to the internet (static route seems not to be working, but maybe just because D-Link is not working and then the router tries the default gateway).

Question
Which settings would be needed on the D-Link to behave like intended?
I downloaded OpenWrt from
(even it's an openWrt link, the forum does not allow me to post it..)
OpenWrt Chaos Calmer 15.05.1
I am using the LuCI GUI.

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read this, if you want to know more about why I am trying this
Background
I have three subnets. router A is a Gateway and offers public internet access. Router B's wan port is attached to a LAN port of Router A. The same holds true for router C. That said, subnets B and C are independent and located "inside" A.
Now certain clients in subnet B need access to certain clients in subnet C. Generally not all clients of B and C should see each other.
There are different methods to achieve this:
Method 1: Static routes on router A.
(the forum does not allow me to provide you the link)
Problem here was, a) I do not really like the solution, as traffic goes through the public network, b) I was not able to configure the firewalls router B and C (on one of them NAT needs to stay active).
Method 2: Connect the two routers directly
(the forum does not allow me to provide the link, would be for method 2 and 3)
Not possible as the routers stock firmware does not support more than two interfaces. I can not play around/mess with those hence no flashing of the firmware.
Method 3: Connect the subnets over an aditional router.
That's where I need help.
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Best regards