Impossible to say without seeing an error message or screenshot of why the connection is failing.
Are you sure your phone is connected to the same network as your Tableau Server? If it's not, are you sure your Tableau Server is publicly accessible?
Well, I am hardwired on network with my company's desktop and using tableau desktop connected to a particular server of the company.
If i talk about the tableau mobile, i am using the wifi network of the company and trying to enter the same server name to which I am connected over the tableau desktop. The error says " Cannot connect to the server".
If you connected your laptop to the wifi, would you be able to connect to the tableau server using the server name you are putting in on the mobile app?
You need to make certain you can actually access the resource from the wifi network you are using on your phone. It's not uncommon for a company to expose a wifi network for staff which basically just has internet and some limited intranet resources but not the full ability to connect to resources like the tableau server itself.