Why related() function can not be used under DirectQuery

The relation is as following: 

The AssetMaster table is pulled as a driectquery model. The other two equipment tables are connected with AssetMaster by "Internal Rate Group ", one to many. I want to toggle a column from Equipment Type table to AssetMaster table by using related function, but there is an error.

What can I do in this case?

Thank you in advance!

Branko