Hello Mark,
There's a bug in Moodle 3.1.0 that doesn't allow submitting HTML, so if you are using this exact version online text plugin is disabled.
To fix that you can update to 3.1.1 or to the latest. Also you can install or update Moodle Mobile additional features plugin, our recommendation is to have both (last version and this plugin).
If this does not help you, please send us your configuration (Moodle version and Moodle Mobile additional features version).
Cheers,
Pau