Search all Sites on Multisite
Currently, you unfortunately can't query all sites within your entires, you will have to separate your results out with a for-loop by site, as @aran did above. 

Craft 3.2 will thankfully fix this by allowing you to pass sites to an array or query sites('*') to return entries from all your sites:

https://github.com/craftcms/cms/commit/d01702278bd84b84392e11da72aa4d0b6f4f0943