OpenWrt snapshots are available for GL-MT1300. I tried one pre-built, and also one that I built, and wireless works - but the 5ghz txpower is very low. Unusable even at 1 meter distance.
After reading some issues, it seems this was a known problem with 7615 driver?
Is it possible to use a blob from gl-inet stock firmware to get best performance and reliability from the wifi? I have read the private driver gl-inet uses is not compatible with LuCI. This is ok as I do not compile LuCI in my images.