Hello  @dcanob

Thank you very much for starting a detailed post and for providing the speedtest and Wifi link speed 

RE300's max Wifi Link speed is 2Mimo*433= 866Mbps @5GHz band, you may test on a mobile phone/ network adapter that supports  866 Link speed/ 2 Mimo  and see if the 5Ghz speed is higher. 

Btw, as Range Extender works in 'half-duplex' mode in extender mode,  it can only talk to the main router or the wireless client device in either way, not the same time, the link speed at physical layer will cut to half. Real 5Ghz speed could be around 50% of wireless link speed and also limited by the actual speed it receives from the router.

If you expect or need higher speed from the range extender and could wire the RE to router via Ethernet cable, AP mode is another option. 

How to connect TP-Link Wi-fi Extender to Router with Ethernet Cable (Access Point Mode）