I have another router based on rockchip/armv8 and running OpenWrt 22.03.3 (FriendlyElec) and the ALFA adapter connects in AC mode without problems and gives good speeds.
The problem seems to be between the mt76x2u linux driver and the ath79/nand router platform Qualcomm Atheros QCA956X ver 1 rev 0.
I've tried everything from using iwd instead of wpa_supplicant on linux, to switching from dhcpcd to dhclient, to trying the router radio without encryption, to trying different channels and no way to get it to connect in AC mode.
N mode works smoothly on both 2.4 and 5GHz.
My wifi setup is very simple and all the other adapters I have 8812au, 8821cu dual band connect to the router in AC mode with the same configuration without problems.
And scan channels on 5GHz works OK but with default openwrt 22.03 installation... ath10k-firmware-qca9887-ct
not work scan on 5GHz band , the -ct packages not work properly to scan on 5GHz band (luci and command line) on router GL.iNet GL-AR750S (NOR/NAND) on latest official release openwrt 22.03.