You've hit quite a problem. There are 2 designs for tenant db. One uses a single db and a FK for each tenant in the tables. This allows you to easily do what you're trying to do. The other way is how you did, with each tenant having their own db. That makes it hard to get data for all tenants. You'll probably have to query each one separately, unfortunately.
Here's a really good video from Tom Schlick at Laracon last year giving a talk on the pros/cons of each method: https://streamacon.com/video/laracon-us-2017/day-1-tom-schlick