The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
found a feature that would close this issue, but don't know the state.
https://github.com/FAForever/downlords-faf-client/issues/1783
https://github.com/FAForever/server/issues/607
Since this is already ongoing for 3 years, i doubt it would be implemented fast.