OpenWrt AP with all wifi traffic out OpenVPN - Same subnet

You may want to mask the wireless key in your previous post. Or better change it in the OpenWrt.

There are certain issues here. You have one flat broadcast domain. So you can have one DHCP server. I am not sure if the DHCP server in HG659 is flexible to allow custom gateway (option 3), so that you can preconfigure wireless clients to use the 3600 as gateway to the internet. If not you can disable the 659 dhcp and use the 3600 dhcp, which can do such things.
If you don't want to do that, you'll have to create a new network on 3600 for LAN and connect the WAN port to the unmanaged switch. There it is easier with the configuration part, but the casting can be tricky, because devices will not be in the same broadcast domain. However it is not unsolvable.

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