When you want to see your email signature on your Android device, you must use the email app provided by Microsoft.
You can install it directly from the Play Store:
'OWA for Android'.
After installing from the App, you tap the app and sign-in with your Office 365 credentials.
Options for Outlook Web App on mobile device
To use Options on a mobile device, tap the navigation icon Navigation, then the gear icon settings .
After you've made the changes you want, tap the checkmark to save your changes and close Options.
To cancel your changes, tap away from the options page.
When you've created an email signature in Outlook Web App on your desktop or laptop, you’ll see the Plain Text version here.
Only the Plain Text version can be used. This is by Microsoft design, so make sure you create a Plain Text version of your HTML email signature.