I have been the happy owner of a GL-AR750S (Slate) for quite a while and recently upgraded to the GL-MT1300 (Beryl).
I mostly use the router when traveling and in “repeater” mode. To optimize bandwidth, connect to distant APs, and to have a choice in local AP channel (when a STA connection is in place, the AP on the specific frequency must match the STA channel), I have opted for an external (e.g. USB) Ralink based WiFi dongle. The limited 802.11n speed is more than compensated by the reaching capabilities of an external antenna (+3db). In order to avoid any conflict with GL scripts/portal, I made sure that configuration items would not align with existing nomenclature (e.g. extconn vs wwan and extwifi0 vs wlan0 etc.). This approach works flawlessly on the 750S. Unfortunately I can’t say the same for the 1300. While the configuration is virtually identical and to some degree works, the WiFi connection over USB is extremely poor. When observing the USB interface, the RX speed is somewhat proper while the TX speed is abysmal and not matching the RX connection (1-6.5 Mbps). In an attempt to understand the problem I have compared the 750 and the 1300 in the same location, with the same WiFi dongle/s, against 3 different APs (Ubiquiti, iPhone, Windows). Issue remains the same. I am currently waiting for an Alfa AC600 to make sure the issue is not related to USB3.0 and 2.4GHZ. Apart from that, everything seems to point at the USB3 driver (the MTK is the one that gets loaded). Any help, hint, and/or direction is welcome.