When accessing Outlook on the web (OWA) through your mobile, email signatures will be assigned as Plain Text (This is by Microsoft design).
You have to view the website in Desktop Site mode to make use of your HTML signature from Xink.
How to request Desktop Site mode on Android
1. Tap the 3 dots next to the URL bar
2. Tap the Desktop Site
How to request Desktop Site mode on iOS
1. Tap and hold the refresh button in the URL bar
2. Tap Request Desktop Site