I have a TP-Link Archer MR505 router (using a 4G sim card) and have purchased a RE330 range extender. Both of these are supposed to be "OneMesh" compatible, which claims to offer a "unified Wi-Fi network". 

I have followed the set up instructions for the RE330 and, however I do it (I've tried via Tether app and using the WPS buttons), the RE330 creates a new network with a different SSID (with _EXT appended to the SSID).

Please can you tell me how to connect the RE330 to my MR505 so that it extends my existing network as stated on the packaging. Thank you.