Jira cloud jql for identifying tests in a test plan (to replace testPlanTests)

[Bobby Reeves]

My organization is migrating from Jira server with the Xray add-on to Jira cloud with Xray. In doing so, we are losing the ability to use certain Xray functions on the cloud so we are having to rewrite queries using these functions. 

To identify test cases that have been associated with a test plan, we would use the following query using the Xray function:

• issue in testPlanTests (key ID of test plan or “filter name”)

This will not work on the cloud. 

Is there a suitable jql substitution to use on the cloud that will return test cases of a test plan? 

I have already contacted Xray support who replied with "most of the JQL functions are not available to our Jira Cloud Add-on. This happens because Jira Cloud doesn't provide us with a mechanism to implement these custom fields and functions safely".

Our migration PM has also reached out to our Atlassian vendor for guidance. No answer as of this post.