Variable in dashboard query is not being interpreted when using PNP4nagios ds

I’m using grafana an PNP4nagios datasource. Everything is working with this datasource.
But I’m trying to add 2 variables within dashboard with this datasource.
1-st variable ‘host’ with query just ‘host’ , This variable is working well and it is displayed in dash board and you can select host from dropdown menu.
The problem is with the second variable. I want to create second variable with name ‘service’, to be able to choose service for the selected first variable host:
The query is simple: ‘services where host = /^$host$/’
But in this query the variablre $host is not beeing interpreted and the request is always to string ‘$host’ , but not to it value of this variable.
How can I make this query to interpret variables in it, and where is the problem ? In query, in dashboard on in datasource ?

Thank you.