Router connection speed

[#1]

Why does my WiFi status  speed keep dropping from 433.3Mbps to 72.2Mbps 

[#2]

The impossible (on a Plusnet connection)  433Mbps result suggests that you are testing using the BTw speedtester. This only seems to work reliably when using either Microsoft's Internet Explorer or Edge browsers. 

Try a different tester such as the Thinkbroadband one.

[#3]

I don't mean download speed, I mean the connection speed from pc to router found under WiFi status-connection-speed

[#4]

It suggests the device is dropping from the 5GHz bands to the 2.4GHz bands. The former is less susceptible to interference/channel overlap and quicker, but it doesn't perform so well over distance.

I think the network map in the router interface may give you some clues as to which band is in use (if not there's a 'Home Network' section that does).

[#5]

Inssider shows

Channel 6 on 2.4GHz

Channel 48 on 5GHz       

5 GHz Wireless Configuration

Sync with 2.4 GHz: YES

Use the same Wireless SSID and Wireless key as 2.4 GHz

I’m no boffin and this means very little to me. I don’t know if I’m routing on 2.4GHz or 5Ghz 

[#6]

If you go into the router advanced network page this will tell you. For the best results many devices work better if the two bands are separated and that which works best is used. When they are synced some devices tend to flip flop between the two, as yours seems to be doing and this can affect performance. To split the bands see https://community.plus.net/t5/My-Router/2-4ghz-and-or-5ghz-best-practice/m-p/1351966#M840