Hi Peter,
Start by leaving out the ASIAIR app in the tablet, and also leave out the Ethernet cable between the ASIAIR LAN port and the extender.
Follow the instructions from TP-Link to connect the WR-902AC to your home network as an extender (don't forget the little mode slide switch on the side of the WR-902). This will involve discovering your home network and give the WR-902 your home network's password.
Once the network connection LED on the WR-902 is lit green, you are good to go. Until then, don't proceed to the next steps. First work to get your TP-Link router to join your home network as an wireless extender.
Now boot up the ASIAIR and wait until you hear a single beep. Wait another 15 seconds just to be sure, and connect the Ethernet cable between the ASIAIR and the WR-902. (After this initial connection, you can just leave the cable attached all the time.)
After the above, you should see ASIAIR appear on your home router's list of connected devices. With some routers, you need to refresh the connection list manually to see newly connected devices.
At this point, the network traffic LED on the RJ-45 connector of the ASIAIR should also flash pretty constantly. This is useful when you are outdoors, to check if ASIAIR is connected to your home network.
This next setup is not necessary, but is useful if you want to use the Samba file server in the Raspberry Pi to download files from the ASIAIR: go to your home router and reserve a fixed IP address to the ASIAIR. In the future you can use that IP address to access your image files on ASIAIR (on macos, the IP starts with "smb://", for example smb://192.168.7.111)
If the ASIAIR is seen by your router, you are all set. Connect the tablet to the home network's WiFi, and launch the ASIAIR app.
Chen