There are some feature limitations with the TB16 and Ubuntu:

http://www.dell.com/support/article/us/en/04/sln305753/dell-dock-wd15-dell-thunderbolt-dock-tb16-support-for-ubuntu-1604-operating-system?lang=en

If changing the "No Security" setting doesn't help, try updating the BIOS and dock drivers as outlined in the bottom section here:

http://www.dell.com/support/article/us/en/04/sln304347/dell-thunderbolt-dock-tb16-driver-installation-guide?lang=en

Have these issues been addressed/mitigated yet?  Or is there any expectation that full compatibility is in the future?  If not is there a dock that is supported that also allows dual monitors with 4k resolution?