It is not surprising that 5GHz wifi is much faster than 2.4GHz wifi. Connecting the router over wifi for both WWAN and WLAN would also have a negative effect.

The speeds also depend on the wifi bandwidth setting. In my urban wifi-congested location, I use only 20MHz bandwidth for 2.4GHz and 40MHz bandwidth for 5GHz., which reduce the speeds even more.

GL.iNet’s advertised speeds are only the theoretical speeds of 300Mbps for the 802.11n 2.4GHz frequency+40MHz bandwidth specification and 433 Mbps for the 802.11ac 5GHz frequency+80MHz bandwidth specification. The real speeds are usually at most 50% of theoretical speeds.