@mag703 

APs don't do NAT - it's a routers job to do NAT. In client mode TL-WA901N serves as a "wireless adapter" to a client device with only ethernet cable connectivity.

If you have a main router with DHCP server enabled you should disable TL-WA901N's DHCP server and choose SmartIP(DHCP) or Static IP address, depending on you preference:

The idea is your client device to have an IP address in your main router's subnet for successful communication:

"Client Mode - This device will act as a wireless station to enable wired host(s) to access AP.

• Survey - Click this button, you can search the AP which runs in the current channel.
• Enable WDS - The AP client can connect to AP with WDS enabled or disabled. If WDS is enabled, all traffic from wired networks will be forwarded in the format of WDS frames consisting of four address fields. If WDS is disabled, three address frames are used. If your AP supports WDS well, please enable this option.
• Wireless Name of Root AP - Enter the SSID of AP that you want to access.
• MAC Address of Root AP - Enter the MAC address of AP that you want to access. When you use the survey function to fulfill the Wireless Name of Root AP, this field will be filled in automatically."

Here's a guide on how to configure TL-WA901N in Client Mode.

  @mag703 

Don't worry about that, just follow the guide I posted.

There's so called Smart DHCP feature which will handle that for you - here a more detailed explanation.

If you insist to control TL-WA901N DHCP server manually just set a Static IP on its LAN within your main router LAN subnet.