How to switch from libinput to synaptics in Ubuntu 18.04
I had the same problem and basically solved it by following Ryko's advice.
sudo apt install xserver-xorg-input-synaptics
... did the trick. I didn't uninstall libinput like Pilot6 said and libinput is still happily handling my keyboard, etc.
If you're running an HWE release (like 18.04.2) then you'll actually want:
sudo apt install xserver-xorg-input-synaptics-hwe-18.04
You don't need to remove any packages, just add this and once it's active you can make adjustments to your taste. (as others have indicated above)
Hint: The suffix "-hwe-18.04" needs to be appended to any pkg that HWE impacts. Read up on HWE and you'll understand why and the scope of the packages impacted.
The problem is not necessarily the missing Synaptics drivers, it is more the fact that the libinput driver settings are purposefully left generic so that the user or hardware OEM can adjust them as desired.
Rather than simply uninstalling libinput and installing Synaptics, you can still add Synaptics but to keep everything working you need to provide an "override" by removing/updating references to libinput specifically in relation to your touchpad ONLY and adding in a rule for Synaptics.
A major part of the problem is that there doesn't appear to be any proper GUI tool and few of the hardware OEMs (Dell/HP/Lenovo/etc) provide customized configs optimized to their hardware outside the possible exception of their own preinstalled Ubuntu versions (Dell provides this option at least). I haven't grabbed the latest Dell Ubuntu image and unpacked it to see if they supply any customizations, but it wouldn't surprise me if they did. If not I definitely think it is something they should see about adding, since the XPS13/XPS15 and their business brothers the Precision 55x0 models are amazingly fast running Ubuntu compared to Windows.
This article by Dell has a good start on how to potentially fix the issue (and doesn't necessarily only apply to their systems or their touchpads), https://www.dell.com/support/article/us/en/19/sln308258/precision-xps-ubuntu-general-touchpad-mouse-issue-fix?lang=en