The connection diagram you provided represents my setup exactly.
Let me explain step by step what I did:
I installed the 4.6.3-op24 firmware on the GL-MT6000. I did a clean installation by selecting not to save the old configurations. After the installation, it's not possible to connect via Wi-Fi as it doesn't accept the password. I had to connect using an RJ45 cable, and after pressing the System > Timezone > Sync button, the Wi-Fi accepts the password and connects. This is not a major issue.
Then, I went to Network > LAN settings and set the router's IP address to 192.168.2.1, which is the IP range I always use for my internal network.
In the Wireless section, I set the following for 5GHz:
Wi-Fi Security: WPA2-PSK/WPA3-SAE
Bandwidth: 160 MHz
Channel: 48 (I chose channel 48 because when set to auto, the GL-AX1800 Flint cannot connect at all. It only connects successfully on channels 36, 40, 44, and 48.)
For 2.4GHz, I used these settings:
Wi-Fi Security: WPA2-PSK/WPA3-SAE
I saved the settings and restarted the router.
Then, I followed these steps on the GL-AX1800 with version 4.6.2:
I held the reset button on the back of the router for 10 seconds to reset it to factory settings for a clean setup.
I set a new password for the router.
I clicked System > Timezone > Sync.
In the Wireless section, I set the following for 5GHz:
Wi-Fi Security: WPA2-PSK/WPA3-SAE
For 2.4GHz, I used the same settings:
Wi-Fi Security: WPA2-PSK/WPA3-SAE
I saved the settings and restarted the router.
After connecting to the GL-AX1800 Wi-Fi, I went to Network > Network Mode > WDS and connected to the GL-MT6000 5GHz network by entering the password. After this, I connected to the GL-AX1800 Wi-Fi and it automatically received an IP address from the 192.168.2.x range of the GL-MT6000, showing that the connection was successful.
However, when I opened https://www.speedtest.net/, it took a very long time to load. Even after waiting for around 3 minutes, the page only partially loaded (as shown in the image I shared) and I couldn’t perform a speed test.
In another test, I connected to the GL-AX1800 Wi-Fi from 1 meter away with an iPhone 14 and again had to wait a long time for the page to load. The result was the same as before.
When I opened fast.net from my computer, the speed results were almost identical to previous tests.
I repeated the test by setting the Bandwidth to 80 MHz instead of 160 MHz, but the results were still the same.
If you would like me to test anything else, I’d be happy to help and contribute to solving this bug.