Install the Android app
The app is required: it is what talks to the phone and sends through the SIM card.
Installation steps
Download the APK
Go to droidsender.com/download/android and download the latest version.
Allow unknown sources
On Android, go to Settings, Security, Unknown sources, and allow DroidSender to be installed.
Install
Open the downloaded file and tap Install. It takes a few seconds.
Grant the permissions
On first launch the app asks for SMS (send and receive), Contacts (optional, for importing) and Notifications (alerts for new messages).
Keep Google Messages in SMS mode
Turn RCS chats off in the phone's messaging app. With RCS on, conversation syncing stops working.