Cudy WR3000 as AP Client - slow download and fast upload

Cudy WR3000 with OpenWrt 23.05 is connected as AP Client (set in LuCI)
to the main router Dasan H660GM (wifi5, GPON 300/300 Mbps)
and unfortunately the download is very slow, but upload is ok.

RX/TX @ 5 GHz:
60.0 Mbit/s, 40 MHz, MCS 9, Short GI
780.0 Mbit/s, 80 MHz, VHT-MCS 9, VHT-NSS 2
Speedtest: 48/280 Mbps

RX/TX @ 2.4 GHz:
54.0 Mbit/s, 40 MHz, MCS 9
270.0 Mbit/s, 40 MHz, MCS 15
Speedtest: 40/150 Mbps

Laptop connected to Dasan or Cudy in AP mode reaches max RX/TX (of course only in 5GHz),
so it looks like OpenWrt issue or an incompatibility between these devices.
Anyone encountered such a strange download problem?

The only thing that comes to my mind is go back to the stock firmware and test download speed,
but I bought Cudy WR3000 specifically for OpenWrt :confused: