Unable to connect to this network

[#1]

Advice please.

I've had an emergent problem which I think is related to the Plusnet Hub Two that we have. 

Here's what I know so far:

1. I have periodic issues where devices (Android/ColorOS, Windows 11 and Ubuntu Linux so far) cannot join my Plusnet wireless network.  Sometimes it works and then sometimes it doesn't, and this has emerged over the past couple of weeks during which I have been trying various proposed remedies.
2. Anything that is on a cable, even through the powerline and even when that is through a powerline wifi-extender is working fine.  It is only the direct wifi connection to the router that is having the issue.
3. For belt and braces I have run a cabled speed check using the BT speed checker which comes in at 70-odd MBps download, which is expected for the connection that we have.
4. Doing the research I have tried: Router on/off, router software reset, router hardware reset, affected devices on/off, devices forget network and rejoin, reset network (W11), reset network adaptor (W11), flush DNS (W11), switch 5Ghz off on router, toggle airplane mode on/off (W11), rescan channels on router.
5. None of the above have stopped the issue.  Right now I am posting this on a W11 laptop that has an ethernet connection to the router that is working fine but the wifi connection states "Unable to connect to this network" when I try it.
6. The wifi extender is just within range of where I am perched next to the Hub Two.  If I drop my ethernet connection I can still work off of the wifi extender connection albeit quite slowly.  This extended wifi has a different SSID and password.
7. The wifi icon for the direct connection to the router does show a dip in strength occasionally but even when showing full strength I still get the "Unable to connect to this network" response.

Anybody got suggestions for what I should try next?

[#2 · Accepted solution]

Temporarily disable wifi security on router; do the wifi connected devices then function?
If so perhaps amend router security type, apply p/word etc and advise if other devices function.

[#3]

Thank you Gel.  I switched to an open network and was able to connect to it.  I then switched it back to secure (same password, just toggled the security back on) and it is still up and connected.  The wireless printer that had been waiting to print something sprang into life so that suggests a broader fix.

I'm going to monitor it for a few hours and check for any relapse but that appears to have done the trick.  So thank you for that.

Any idea as to why that worked?

[#4]

For completeness I forgot to list that I also tried changing the Network Mode between 1,2 and 3 and all of the modes had the same issue.  Just in case someone else find themselves in my position and reads this thread.

[#5]

Damn - spoke too soon.  This morning we are back with the same issues.

There goes my Sunday.

[#6]

OK - reset all powerline adaptors for something to do and redid the security trick.  It is back up for now, i don't know what got fixed or how long it will stay up.

The only thing I did differently (aside from the powerline reset) was to retire some Fujitsu meshhu wireless extenders on the off chance that they might be causing some issue.

Apart from that I have tiptoed away from the router and will try not to anger the wireless gods in case it goes off on one again.