WIFI rate negotiation is always multi-faceted. In your screenshot, the speed of AX1800 in one direction is only 197M, while the speed in the other direction is 866M, which may be the optimal speed negotiated by the algorithm according to different hardware characteristics. You can actually test the speed, the page display may not be accurate.
You could try building a Kernel version: 5.4 for Flint from the wlan-ap-5.4 in infra-build, since the Slate AX is the same, I’ve noticed quite a difference between the two, 5.4 missing some 802.11 feature set but better performance wise for 5GHz.
Try the 0.99.8 version of SmoothWAN from here Releases · SmoothWAN/SmoothWAN · GitHub SmoothWAN-0.99.8HF1-TEST.3-GLiNet-Flint-WebUI.img.tar
Set it up like the usual OpenWrt device (ignore the apps guide etc)
That one is using latest kernel. It has a quirk with the 5Ghz adapter needing to be disabled before Scan button works, else the results will be empty.
The problem is whatever setting i have tried on flint it always being limited to 20mhz,
That’s why i said at the first time that with the same exact configuration either FT being enabled or not, flint always be limited to 20mhz
Only the flint openwrt built by solideus that exact fw gave me full 80mhz.
I’ve been spending like 2 weeks to just flash and reflash all kind flint custom and official glinet, and none gave me full 80mhz on AC or AX