I cannot get my HP Deskjet 3520 to connect to wifi.

[Penny_Printer]

My printer worked fine up until about 3 weeks ago when I had to reset my wifi system.  This is Virgin Broadband augmented with BT Whole Home and the printer previously connected successfully to BT Whole Home.  Now I just get an error message - "Error ... progress stopped"  each time I try.  The wifi is "on"  although it keeps going off and I have to turn it back on again, and Wireless Direct is "on".

I have connected the printer to the laptop with a USB cable and successfully printed using that.  

I have been through the Print Doctor thing, and also all the suggestions from the HP Smart app, which believes the printer to be switched off.  It isn't.

Any thoughts?  

[Penny_Printer]

Some more information on what I've been trying:

- downloaded latest drivers and firmware and installed 

- tried HP Smart and HP Doctor to connect and followed all their instructions  

- successfully printed a Word doc via the USB cable.  

- other devices are working fine on that router

- other devices which previously worked with the printer no longer work (e.g. iPhone) as they can't find it

- tried the 2.4 ghz temporary re-setting on the router in case it was that (although they are separately listed on it) 

Message I repeatedly fail at is:

"Issue: Incorrect wireless password.
Solution: The WPA security passphrase you configured on your HP printer does not match the WPA passphrase used by the wireless router. Configure the printer using the correct WPA passphrase."

It is not incorrect - everything else is working fine on that password and it has never been changed (notably it's the one that worked fine three weeks or so ago).  

Any thoughts anyone?  Why does it not recognise a correct passphrase?