Something really crappy happened to Ubuntu between 18.04 and 20.04. When I upgraded, I discovered that my printing was broken. Network printers were found, and they even worked for a while, but they soon broke, the computer unable to communicate with them.
Here’s how I fixed it:
Go to the CUPS admin interface in a web browser (http://localhost:631/) and use the dialog to add a network printer. Any protocol will be fine, you’re going to change it. Make sure you rename the printer to something you’ll recognize as distinct from the name CUPS has given it. Select a driver like normal, then save.
NOW, open your Printers dialog. Find the printer and select Properties. Under Device URI, hit Change. Under Network Printer, select AppSocket/HP JetDirect. Specify the IP address of the printer for the Host field. Hit Apply.
On my brand new install, I lost the Print Test Page option. No biggy. Just open a file and print to this printer, the one you renamed. You’ll see the default CUPS name printer on the list, ignore it. Just print to the renamed one you just tweaked.
If your luck is as good as mine, it will work PERMANENTLY. Note that if you go back to the web-based CUPS admin and delete the original (non-working) printer, your renamed printer will vanish as well. So get used to seeing unusable printers on your print dialog.
This technique has worked flawlessly for my two Brother printers, an HL-L51000DN and a MFC-L3750CDW.