Powerapp Sharepoint based form with drop-down using excel table items

I have created a SharePoint list with a.o. Choice columns. One of those columns is called GTM. Linking this table to a powerapp form works well and the GTM datacard is shown as a drop-down item. All ok. Now I want this drop down to be linked to an excel table, called GTM_Ex. That also worked out fine. Now when I submit the form, the selected value is not passed to the SharePoint table. The datacard Update is pointing to the drop-down datacardvalue. 
Alternatively, I removed the drop down and inserted a new (classical) drop down item in the datacard. Linked it to the excel. Made sure Update of datacard is pointing to its selected value…but again, selected value of the drop down did not pass to the SharePoint after an updateform command. 
anyone any idea?