Big Sur 11.5 seems to have broken my WiFi connection

Hello,

First time posting here, never had a reason to post before, I hope someone can help me!

I have a iMac (Retina 4K, 21.5-inch, 2017)

Since upgrading to Big Sur 11.5 a couple of weeks ago my Wifi has not been working. It started dropping out  intermittently but came back almost immediately. Then it stopped reconnecting, only returning if I restarted the machine. Finally it refuses to connect at all, the wifi icon is greyed out with a line through it and remains stubbornly that way, even if the button is blue and “ON”. Same for trying to turn it on via System Preferences. 

 I CAN connect via ethernet or using my Iphone’s hotspot, but only if my phone is connected via USB. 

The WiFi router is working perfectly with all other devices in the house so I’m 99.99% sure the problem isn’t there.

I’ve looked at and tried various solutions. (I hasten to add I found these in various guides online and just followed them, not really knowing what I’m doing) 

1. Rebooted Mac and Router. Going into the routers settings and removing my mac from the network and reconnecting worked for a brief ten minutes or so, but nothing since.

2. Unplugged all USB devices -  I had an apple wired keyboard and generic wired mouse attach plus my time machine drive, replaced the keyboard and mouse with the bluetooth version that came with the machine : made no difference.

3. I deleted Intego Netbarrier and turned off Virus barrier. - no effect.

4. Reset NVRAM, PRAM  and SMC - Seemed to work for a while then disconnected.

5 Made the mac recreate a wifi configuration by deleting “NetworkInterfaces.plist” “com.apple.airport.preference.plist” “com.apple.wifi.message-tracer.plist” “preferences.plist” from  /Library/Preferences/SystemConfiguration/  rebooting and trying to connect. Again, this worked for a few minutes but soon disconnected again and hasn’t really  worked since.

6 Created a new Network Location with custom DNS -“8.8.8.8 and 8.8.4.4” and MTU (1492 and various others) Worked for a minute.

7. Used ethernet connection to install 11.5.1 - no difference.

8. Used ethernet connection to Restore from Time Machine back to the day I installed 11.5 - but it reinstalled 11.5.1 and the upshot was my machine is working faster now but still no wifi!

Hope someone can help, I know enough to navigate around and delete files etc, but if things involve the terminal etc I have no knowledge whatsoever.

Thanks in advance!