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
A. For iOS 12 and earlier.
1. Tap and hold the refresh button in the URL bar.
2. Tap Request Desktop Site
B. For iOS 13 and later.
1. Tap the "aA" icon on the right side of the URL bar.
2. Tap on "Request Desktop Website" on the drop down menu.