@David-Wong 

It would be best to test your phone on other networks to see what it is fully capable of reaching as far as speeds, same for the computer. Make sure the phone does not have a VPN connection enabled.

What you can try also is to test on both 5G bands to see if you get a difference in speeds. 

The router should have DFS channels available so try setting either of the 5G bands to that. When bringing up the channels list certain channels should have (DFS). What may happen is the router will actively scan to see if there are other existing DFS channels, so if you notice it reverts to a non-DFS, then that will be why.

Along with the modifying of the channels, also try modifying the channel width to a higher value.