Hi,
The workaround is on the secongf the page. I'll post a copy:
Hi all,
Since many of you asked, I will share this workaround. On the first place, please keep in mind that the reason I haven't share this is inconsistency.
In some mobile browsers, you have Request desktop site option. Please try to find the option for your browser or research which browser support the option and install it. After that, go tohttps://outlook.office.com/owaand activate the option Request desktop site. Now, you have desktop version of Outlook on Web. To add a signature, click on Settings gear in upper left corner and select Mail. On your left sidebar, in Layout section, you can find Mail signature. Try to add your HTML signature here and test it from Outlook on Web application.
Due to issues with some mobile phone browsers, this solution is inconsistent. In my tests, sometimes I couldn’t access the desktop site. This wasn’t related just with Outlook web App. With lot of other web sites, mobile view has priority in mobile browser. I can support just for Office application, so I could give you general suggestion to try to open the web app from a private (incognito) window, or to clear cache from your web browser (please check how to perform the action if you decide to try it) if the browser keeps with opening Outlook on Web for mobile phones.
Best regards,
Nemanja
So this is only workaround I am aware of.
Thanks for your understanding,
Nemanja