Orbi RBR40 L2TP Odd Behavior

Strange one here. I connect to a small business via L2TP VPN. My iOS devices connect just fine. My Mac OS X and Windows 10 devices cannot connect. 

This throws me. Why are the mobile devices working while the full-fledged devices aren't? Obviously, I'm able to reach the remote server from those devices, so the Orbi appears to be doing what it should.

Any clue?