Hi,
I have the following configuration:
TPLINK N750 (TL-WDR4300)
Powered by LuCI openwrt-22.03 branch (git-23.093.57104-ce20b4a) / OpenWrt 22.03.5 r20134-5f15225c1e
I have configured the client mode on the Atheros AR9580 802.11an radio.
When I connect wirelessly with my laptop to the client network (with 5GHz support), the internet speed is about ~300Mbps.
When I connect with a wire to the OpenWRT router (that connects to the client network), the internet speed is about ~50Mbps.
Why? If I understant correctly, by configuring the client mode on the "an" radio, it should connect it in 5Ghz mode, so it should also provide similar speeds as in direct connection.
Signal level is low at -67. You want to be -60 or higher for best speed. This reflects in the MCS being only 120 Mb. This is the raw rate over the radio, the usable throughput is half that or less as you have found.
The hardware in the WDR4300 is an standard (WiFi 4) which supports MCS rate up to 300 Mb (possibly 450 in one direction if the AP is 3x3). The direct to PC link is probably using ac (WiFi 5) or ax (WiFi 6) which are faster standards.