I have a TL-WN722N v1 connected to a RE355 range extender - both from TP-Link. However they only establish a 72 mbps connection via 11n - i.e. one 20 MHz line. Both the TL-WN722N and the RE355 should support a 40 MHz line, resulting in 150 Mbps. The router is setup to use 40 MHz and if I connect the TL-WN722N directly to the router, it connects at 150 Mbps just fine (The signal is just too weak - hence the need for an extender).

I cannot find anywhere to configure the RE355 to use 40 MHz? The configuration interface is very minimalistic. And shouldn't they be able to negotiate a 40 MHZ connection automatically?