Do what SimonBrackley said. Google "Apkpure" on your smartphone (need android afaik). Enter the site and search for "Shimano", select the "E-tube Project Cyclist" app, scroll down to "Previous versions" and select "more", select/download "V3.4.3", find the downloaded file on your phone, open it, let it install, done! I did NOT need the Apkpure App to install the download, even though the site tries to tell you that. Once installed, E-Tube V3.4.3 appeared in the Google Play Store library like a normal app. You should disable automatic updates to prevent it from updating. When you start E-tube 3.4.3 it will try to update to the latest version, so cancel that update. The Light On/Off option is in the Drive Unit customization.
I'm so happy I found this. I used an old phone to download it since I was afraid of malware but so far so good.