Fluctuating WiFi speeds / any analysis tools?

[#1]

Am on 150 Full fibre, and usually get that on hard wire.
I think (terrible memory) I used to get 80-100 Mbps on WiFi
Do live in a built up area (London)

Last night watching a film it suddenly buffered.
Checked speed test on my phone as I was only getting ~ 1Mbps
Restarted Router - no change
Thought it was Internet as a whole - did support "HELP" then "REFRESH" - no change so went to bed.

This morning tested wired speeds and was getting the expected 140-150Mbps

Wifi - Mid morning I was getting - 30-50 Mbps (5GHz)

Do some testing on laptop and Phone (pixel 7a) and check Wifi channels using Android App - WiFi Analyzer.
Do various tests
- Change Wifi channels on router 
- Switch 5GHz on/off to default devices to 2.4GHz
- Move router to a better spot.
(Am 1-2m away)

For most of that was getting 20-30 Mbps, occasionally up to 50

After doing some reading forums etc - speeds drop again - sometimes max 10Mbps on the laptop, or on the phone occasionally <1 Mbps (5GHz) !
Just Off/On wifi on the phone and now getting ~50Mbps
Then 2 min later 20Mbps

It's very odd and fluctuates lots.

I can only imagine that either the hub is on the way out, or there's something new in the area occasionally chucking out very broad spectrum (2.4 + 5GHz) interference.

Anybody know of any (free) analysis tools that could help me pinpoint what the issue is ?
Or any specific steps to try in my testing?

[#2]

Have the old work DrayTek Vigor 2860ac in the attic - I might give that a go!

[#3]

Done the initial setup on the Draytek - and my phone is giving me 150 Mbps.
Wonder what was up with the hubTwo
Then again - the Draytek Wifi signal is significantly stronger than most others around me (other than 1 VM SSID interestingly)

[#4]

Fluctuating wifi speeds are a symptom of the device switching between 2.4 and 5Ghz signals.

On the Hub two you can see the connection speed by checking the event log, navigate on the Hub two

Home >Advanced  settings >Technical log >Event log. select category 'Wifi'

RSSI is the signal strength of the connection, it is a negative figure the lower the number the stronger the signal.

A recommendation would be to try selecting a less busy channel, as displayed by Wifi Analyser.

150 Mbps is the Pixel 7 connected to a 5GHz signal with the fastest connection for your broadband speed.