I've worked with a couple of customers that have used Zebra tablets, though none with the ET40AB specifically. Where you're having problems is unlikely to be device related.
First step is to ensure you have downloaded the appropriate Maximo Mobile application. You need to use the white background app IBM Maximo Mobile since you're on MAS. There is a black background app that is designed exclusively for 7.6.1.X.
Then you need to provide the full Home URL of your MAS application, including the workspace. Some people will use a shortened URL like:
https://home.myapp.mycompany.com
This does NOT work with Maximo Mobile because it builds URLs out of this and is expecting to see the workspace. For example, if your workspace name is MAS and your URL is like the example above, you would use this:
https://mas.home.myapp.mycompany.com