Recently the lower two touchpad buttons on my Dell laptop have stopped working. There is a second set of touchpad buttons above the touchpad under the space bar and these are working as expected. However, I'd prefer to use the lower buttons as usual, as this is more ergonomic and what I'm accustomed to. If I plug in a USB mouse then that works as expected. It's just the lower touchpad buttons that have gone awry.

Can anyone suggest what I can do to diagnose the issue? Are there drivers that I can reinstall or upgrade that may have an effect?

Dell Latitude 7480, Ubuntu 18.04 LTS

Thanks in advance for any comments or suggestions!