OK PROBLEM SOLVED !!!
After some research I have opted for the "Client Bridge" configuration for my Linksys,
Nearly no change was required on my main gateway (the ZyXel router) except changing the security mode from WPA to WPA2
The rest is described step-by-step in the procedure below
As a reminder, I have now:
A ZyXel router Second floor acting as a gateway
A Linksys router First floor acting as a client bridge
No cable between them, they communicate wirelessly.
I have in total 9 wireless devices i the house; all with 5 bars (max reception) and improved surfing and download speed,
It was worth the effort... espacialy when adding only an old router to my configuration
Hope it's going to help someone else out there
And thank you for those who took the time to read my posts and to "cup" who put me in the right direction this morning !
Client Bridge PROCEDURE
First of all, do not even try to do client bridge with dd-wrt SP1. It won't work.
The router that you are trying to connect to is the primary router. The router you are configuring is the client bridge(CB) router. You don't need to worry about the primary router as long a s you set your CB to the SAME subnet. What this means is, if the primary router has an IP of 192.168.A.X, you need to set the CB router to an IP of 192.168.A.Z. Most primary routers will be at 192.168.1.1. So, when you are configuring your CB router, set it to 192.168.1.9
With that in mind, here is how to configure it.
1. Download a recommended build for your router. DO NOT USE SP1. If you are using a broadcom router, read the peacock thread prior to starting. http://www.dd-wrt.com/phpBB2/viewtopic.php?t=51486
Use one of those recommended builds.
2. Do a HARD reset on your router.
3. Connect a cable from your computer to the LAN port on your router.
4. Set your computer to a static IP address of 192.168.1.9
5. Set your browser to 192.168.1.1 and open the dd-wrt webgui.
6. You should be asked to change your password and username. Carefully type these in. If you aren't asked for a password and username, do a hard reset, this time doing it properly! Hit change password.
7. Go FIRST to wireless, wireless security and enter the security type and key that matches your primary router. You must use wpa2-aes for things to work reliably. The only other that works reliably is wep. You might have to change your primary router to a security that works with dd-wrt.
8. Hit SAVE, (or APPLY if you wish to)
9. Go to the wireless, basic settings page and change the wireless mode to Client Bridge.
10. Your wireless network mode should be set to the same as the primary router.
11. Set the wireless channel to match your primary router channel if there is a channel selection available.
12. If you are using N only or Mixed with N, set your wireless channel width to match your primary
13. Set the wireless network name to exactly the same as your primary router. Make sure spelling and capitalization match.
14. Set Ack timing to 0 unless you have a long distance (>300 meter) link (Optional with broadcom routers)
15. Hit SAVE at the bottom. Then hit APPLY.
16. Check to make sure all the configurations, including the mode, saved and the mode is still client bridge. If any changed, fix them, and save again.
17. Goto Setup, basic setup and enter a router Local IP address of 192.168.1.6. Leave subnet mask at 255.255.255.0.
18. Set the Gateway IP to your primary router. (Likely 192.168.1.1)
19. Leave Local DNS blank
20. Check Assign Wan port to a switch, if you wish to.
21. Change your timezone and DST to match where you are.
22. Hit Apply.
23. Set your browser to 192.168.1.6 and login to your router
24. Goto Security, Firewall. Under Block Wan Requests, uncheck everything but "Filter Multicast" (Leave Filter multicast checked).
25. Hit Save
26. Disable SPI Firewall
27. Hit Apply.
28. Go to setup/advanced routing and change the operating mode from "gateway" to router. Hit Apply.
29. Set your computer back to auto IP and auto DNS.
You are done. Configure any other settings you wish at this point. .