After every sale ( any type ), sale type switches back to the default sale type. In order to make Dine In as a default sale type, you must edit the register. Follow the below steps to make the Dine In mode as the default sale type.
1. Click Settings on the navigation bar and select Stores & Registers from the drop-down
2. Click Edit on the register line
3. Scroll to the Default Sale Type and select Dine In. Click on Edit Register to save the changes made.