@poqh 

Try testing with all available channels rather than auto.

If you have no AX devices try switching the mode to a/n/ac to see if you get any difference in speed.

I do have an AX device.

1. WIth AX only, the maximum is still below 500Mbps.
2. With ac only, no different.
3. Changing channel? No different.
4. With QoS, nothing better.
5. Move close to the router? It doesn't help.
6. Factory reset? The same result.
7. Upgraded firmware? -- It's even before I tested the speed.
8. The problem only exists on 5G -- no, 2G is even worse than average AC router, just 100Mbps limit.

So, anything else you want me to try? Don't forget that it can reach 1 Gbps for LAN, so it does not the internet problem.

@poqh 

May seem odd, but if you are using the Tether app, uninstall it and reboot the AX50 and retest the speeds.

I should rest my case. Due to the experiment from https://www.gearprimer.com/technology/testing-apple-macbook-pro-wifi/, I guess 2x2 AC MU-MIMO can reach only <500Mbps limit. So it's my fault that I don't know about this.

For whom might ask, max bandwidth for this device (with MacBook pro 16") is ~610Mbps (AC 2x2 80Mhz) when you sit next to it with DFS channel selected. And AX3000 network adapter cannot do more than 500Mbps(80/160Mhz) with 5m far and thick wall. So it just better for only your ping, not your download speed. If anyone wants more download speed you should buy 4x4 MU-MIMO. I suggest Xiaomi AX3600 which just 100$ now with a discount as an alternative.