System: Ubuntu Mate 19.10

The printer in question is installed but I can't seem to keep "Enabled" & "Shared" checked.
One or the other, or both, depending,
When the print job is sent they un-check.
At wits end

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I suggest you start again.

Be aware that to successfully edit or add files to /etc/cups....you need root powers!
None of this sudo stuff.

use your current web browser to go to this link please
http://localhost:631/printers/
click on each model you see there.....assuming there is more than one...but for each faulty one....
click on the pulldown for Admin and select delete printer.
Your web browser must allow popups ....for this site....meaning the localhost site....in case you have all popups blocked.
Once you see the popup....username=root
passwd is the real password for root.

once you have deleted all bad cups setup... start again
click on this link
http://localhost:631/admin

click on Add Printer button.....DO NOT click on Find printers....its a common mistake by some.
popup....root and root pw please
Now ....for this session....you have the power to detect printers and add their configs to /etc/cups
just go thru the steps to set them up.

You will see a box to tick for shared and not sure about "keep enabled"
good luck

edit

here is what my WL canon looks like in the web browser once you have set it up
used for ink levels mainly
https://imgur.com/42dz88s
https://imgur.com/XyVRemw
and what my canon looks like in the web browser after set up

PS no one shared my PC or printer

According to openprinting.org, this printer should work and play well with Linux.

I second aus9's suggestion to check what CUPS says about the printer's status.

Thank you aus9 for the detailed procedure. I will follow up later today or tomorrow.
Popups: So far I've stuck with the default browser -- Firefox. I was mostly
following directions in the (popup) troubleshooter. Since I'm the only user on
this unit I didn't realize I needed to differentiate between sudo & root.

Lesson learned(?). Time will tell :-)

I spent yesterday afternoon, off and on, experimenting with this printer thing.
I managed to get probably every error message known to man.

Rather than give up I started back this morning. Started with logging into the
MG3520 and disabling WSD. Not sure if that helped or not.....
One attempt managed to generate a different message "unable to write print data,
broken pipe".

Finally, success, using lpd option. I was able to print a test page -- plus it flushed two other
test page jobs that I hadn't deleted.

Question: Is the "queue" significant? Hate to change anything at this stage.

Thanks again for the help.

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yes its sequential....cancel old jobs if you no longer need them.
Your status image....looks like you have resolved your Original post?

Yes, I 5hink it's resolved.