Upgrade problem, old database not on db list in 2015 dev env

MaW

I am trying to upgrade for the first time.
I have a multitenant customer data database which I want to upgrade and move to new 2015 server.
The current setup is 2013 R2.

I have imported the 2015 license to the tenant database. After that, I have backed up the db and moved it to the SQL server on the new setup. Next step is to open the database in the 2015 dev environment. But when I try to open database, I do not see the database in the list. I am pretty sure, that all permissions are set up the way they should on SQL server.

I even tried to create a new SA user on the SQL server and give it the necessary rights to the db's. But only the 2015 databases are on the db list, when I try to open another database.
Where do I go wrong?