I subscribe internet is 800mbps, my Wifi unable to provide me the 800mbps

First at all, you have to be ensure your device are support Wifi 802.11AC Wifi5 or AX Wifi6, and your router is 802.11AC or AX capability.
for ISP lower then 500mbps 802.11AC or Wifi5 is recommand, for ISP grather then 500mbps 802.11AX or Wifi6 is recommanded.
in the meantime, you have to ensure that you are connecting to your 5Ghz instead of 2.4Ghz wireless channel.

The two big differences between these upending internet technologies, 2.4 GHz and 5 GHz Wifi connections are - speed and range.

A wireless transmission at 2.4 GHz provides internet to a larger area but sacrifices the fast internet speed, while 5 GHz provides faster speeds but restricts itself to a smaller area

2.4 GHz -
The 2.4 GHz frequency of the wifi router offers the wifi user a wide coverage area and is better at penetrating solid objects with a maximum speed of 150 Mbps.

On the other hand, it has a lower data range and is highly prone to interference and disturbance.

5 GHz
The 5 GHz frequency facilitates the customer with a higher data range with negligible interference and offers a great deal when it comes to the internet speeds for wifi for home.

Contrary to this benefit, it has a narrow coverage area and is not a successful band when it comes to penetrating solid objects.

Channel Width
Channel width dictates the amount of bandwidth used within the radio spectrum during transmission. Using more bandwidth can lead to faster speeds, but also increases the chances that you’ll be interfered with by someone else also using the same area of the radio spectrum. WiFi bandwidth is measured in megahertz (MHz). The minimum channel width needed is 20 MHz. 40 MHz and 80 MHz channel width are also supported on some devices.

in most of the circumstances is because user did not properlly configure the channel width, below is the guide to change the channel width to 80mhz inorder to receive the ideal of connection speed.

Slow internet speeds can be caused by a number of things. Your router could be outdated or it could be too far away from your TV or computer, for example. Those fixes may be as easy as restarting your modem and router or upgrading to a mesh network. But another reason for your slow Wi-Fi could be bandwidth throttling.

STEP 1
First, troubleshoot your slow internet connection
Connect your complter or laptop via cable connection VS Wifi connection

STEP 2
Test your internet speed
Test your computer internet speed and test again from your Wifi connection (it will be best if test on same laptop)

STEP 3
Fix your Wireless setting
check your wifi connection

STEP 4
configure your Wifi setting
access into your wifi router and perform the configuration check

Andrew replied at 2022-3-18 12:41
Slow internet speeds can be caused by a number of things. Your router could be outdated or it could  ...

I had a similar situation: the internet, which was promised at 1000 megabits per second, suddenly started slowing down. I thought the problem was in my equipment, checked my router settings and made sure I was connected to 5 GHz. But a speed test showed that I was getting less than half the promised speed.
This was especially critical for me because I work on large construction projects, and I need a stable connection to work with specialised software. I contacted my ISP and they confirmed that they were having temporary network issues.  After they fixed it, the speed was back to normal and I can work with heavy files and 3D models again. It's important to not only check your devices, but also to keep an eye on your ISP.

Ivan Smirnou replied at 2024-11-22 06:05
I had a similar situation: the internet, which was promised at 1000 megabits per second, suddenly s ...

Yes Ivan that's true, is important to see all the possible causes including the ISP.
Many thanks for share this with us!
Regards