UPDATE:
I bought a generic USB AC1200 wifi adapter with MediaTek MT7612U chipset for $20 from Amazon. After installing the kmod-mt76x2u package on my GL-AR750S and plugging it in, a new radio showed up in LuCI → Network → Wireless (does not show up in the GL.iNet UI).

I was able to configure the new 5GHz radio in LuCI as a LAN Access Point with the built-in 5GHz radio as a WAN Repeater, instead of using the slower built-n 2.4GHz radio.

EDIT:
It seems better to configure the new 5GHz radio as the WWAN Repeater and the built-in 5GHz radio as the WLAN Access Point.
The MT7612U wifi adapter gives improved speed and reliable connection because I get frequent wifi connections when using only the built-in 5GHz radio for both WWAN and WLAN.

2 Likes