You need to urlEncode the filter part:
'https://gateway.eu1.mindsphere.io/api/assetmanagement/v3/assettypes?page=1&filter=' + urlEncode('{"name": { "contains": "CNG"}}')
Did you try to excape the curly brace with another curly brace (so use {{ instead of just one {?
Regards,
Ronald
[EDIT]
Just to be sure. Did you try filter={{"name": {{ "contains": "CNG"}}}} ?