There may not be enough info to diagnose here, but first:
If you're configuring as a dumb AP, you must have a cable connecting the LAN port on your main router going to the LAN port on the dumb AP. If you connected it to the OpenWrt WAN port, the dumb AP functions won't work.
In addition, if you want to access the OpenWrt dumb AP, the LAN network must be the same as the main router -- you can manually assign the IP as long as it is not in the main router's DHCP range and is not otherwise being used. If the OpenWrt device itself needs internet access, be sure to set the system DNS and gateway consistent with your main network (this is not required for the dumb AP to work in general).
If this doesn't solve your issue, please post the contents of these files:
/etc/config/wireless (you can obscure the passwords)
and also please let us know what the IP address (or network definition) is for your main router/LAN.