Can't scan without USB connection

Are you able to print to the printer?

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

Try the following: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

If you upgraded to Tahoe, you probably need to reinstall the Canon MG3520 driver(s). Check the Canon support site for possible driver updates.

if you are scanning from the canon software -- go to canon and check the macOS SYSTEM REQUIREMENTS for your scanner (and google if others are having compatibility issues and how they fixed it, the solution)

it probably needs a software update to work... Tahoe 26.1 is still the bleeding edge...

PS: I presume you mean it has never worked since updating to macOS 26x

Try using the Image Capture Application for scanning.

Image Capture User Guide for Mac - Apple Support

If Image Capture doesn't recognize your scanner, reset the iMac 's printing/scannig system.

Reset the printing system on your Mac to solve a problem - Apple Support

Solve scanning problems on Mac - Apple Support