Sadly, on Android devices, nothing seems to work.
The Gmail app on Android does not read HTML signature from G Suite so it has to be edited by yourself if you wish to attach your plain text signature to your outgoing email messages. Learn more.
Thankfully, there is a solution, even if it may seem a bit severe: Switch to iPhone! Or ask Google to provide better support for signatures for Gmail on Android devices.
We have a feeling that for you Android users out there, this will be the preferred means of protest!
Pending Feature Request with Google.
For the meantime, please use your Gmail via the web browser on your android mobile to send and receive email.