I have the ES File Explorer and the DU Recorder apps installed on my phone, but they cannot be found in the Play Store anymore. So, how can I uninstall those apps?
Google Play Store is not necessary for installation or uninstallation of apps.
All your installed apps are shown in the app list in Android settings. There you can also uninstall apps.
On most Android versions you can also make a long-touch on an app icon on the launcher and then open the App info page where you can uninstall/disable an app.
You should try using adb, I recommend using it on a Linux environment since (for me at least) It's easier to configure.
sudo apt install adb
Then connect your tablet, be sure that you have your device with USB Debugging enabled. then, type.
It should connect without a problem, then you have to identify the package you want to delete with.
pm packages list <the name of your app or a good hint without the arrows>
You should see a result or a list of names that will look something like these
Once you are sure about the package name of the app that you wnat to uninstall you have to tun these command.
pm uninstall --user-0 <the package name without the arrows>
On the other hand, you should be able to uninstall any app you installed just drag and drop... unless is a deprecated google app like google+, and even with this method yo won't be really uninstalling anything, but for your eyes the app is going to disappear.