SharePoint 2016 search - Remote server returned an error (401) Unauthorised

I have two SharePoint 2016 farms. Both are running SharePoint search services. I've published both Search services and I connected to from each farm to the other farm. The connections setup with any errors at all. 

When I add a search werb part to a page that uses a result source that used Remote SharePoint as a protocal I am getting the following erro in the web part:

1¾System.Net.WebException: The remote server returned an error: (401) Unauthorized. at System.Net.HttpWebRequest.GetResponse() at Microsoft.SharePoint.Client.SPWebRequestExecutor.Execute() at Microsoft.SharePoint.Client.ClientContext.GetFormDigestInfoPrivate() at Microsoft.SharePoint.Client.ClientContext.EnsureFormDigest() at Microsoft.SharePoint.Client.ClientContext.ExecuteQuery() at Microsoft.Office.Server.Search.RemoteSharepoint.RemoteSharepointEvaluator.RemoteSharepointProducer.RetrieveDataFromRemoteServer(Object unused) at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at Microsoft.Office.Server.Search.RemoteSharepoint.RemoteSharepointEvaluator.RemoteSharepointProducer.ProcessRecordCore(IRecord record)

When I searched for the above error I got to articles like this about SharePoint 2013:

https://social.technet.microsoft.com/Forums/ie/en-US/292534d0-f87e-4488-9e13-6a1160bd4398/regarding-sharepoint2013-hybrid-search?forum=sharepointsearch

I've double and tripple checked my certificates and they seem to be ok. But the error doesn't go away.

Has anyone got this to work with SP 2016?