Working Offline in Outlook used to be solved by just clicking a button in the Send / Receive tab. This message appears on the lower right border when Outlook is Working Offline but it is not a button, only a status message.
If you just installed Office 2016 and Outlook is stuck in offline mode you may be having difficulty trying to get it back into online mode. This is because there is no longer a button in the Send / Receive tab like in Office 2013.
Luckily there is an easy fix! Just reboot your device and the problem will solve itself. You may be prompted to log into Microsoft services on your computer if you have an Office 365 or Exchange online hosted email account. Fill in all passwords when required and you should be back in business.