DBATools offline install - export-dbainstance was not found
You need to import the module using import-module dbatools 

See Properly importing the module


  Specifying the path to the .psd1 file


hope that helps you !

Also, what Scott commented is documented in the same link you have (just read the page carefully to see if you are not missing any steps):


  Once you’ve extracted the folder, make sure you rename it from dbatools-master or dbatools-development to plain o’ dbatools.