Hello,
but your router is not Dual-Band? What is the wi-fi standard used by the router, 802.11n, 802.11ac or 802.11ax?
Are you very far from your phone compared to the computer where you configured the mobile hotspot? If you move the phone near the computer, does the situation improve or does it remain the same?
If possible, connect to the hotspot via a laptop or other device other than the phone and check the speed that is detected
You have to try changing the driver of the wireless network card as I already suggested to you earlier.