Oh dear. While much has been made of Dell fixing the issues affecting the XPS 15 9570, it appears that the company has broken an important feature for its predecessor, the XPS 15 9560. There have been multiple reports of BIOS 1.14.2 breaking HDMI out for some machines, rendering their HDMI ports unusable.
https://www.notebookcheck.net/Some-Dell-XPS-15-9560-laptops-are-experiencing-a-severe-BIOS-related-HDMI-problem.420326.0.html
I had the same issue on my Precision 5520 laptop. Rolling back to the previous BIOS version brought the HDMI port to life again
My battery life has dropped dramatically since the latest bios update. What was 6-8 hours is now 4 hours, maybe 2 hours. XPS 9560
The XPS 15 9570 is still not working as well! They won't work with an external display if the monitor is old. We have labs with monitors with DVI, VGA and have HDMI to DVI cables on them for people to hookup laptops and the XPS 15 9570 are the only ones not working. Dell basically told us to go pound sand because of the age of the monitors.
9570 have issues as well! www.dell.com/community/XPS/XPS-15-9570-not-working-with-2008-E228WFP/m-p/7293664/highlight/false#M28030
Argh. I just tried using my HDMI port this morning on my xps 9560 and no dice in either windows 10 or Ubuntu 18.04.
I recently updated my bios at I typically do, and verified I do have version 1.14.2. >:(
I have the same issue too on my XPS 9560. The HDMI port stopped working after the update and the battery seems to drop faster than it did before while the laptop is just barely a year and few months old.
I guess try not to fix your computer if it's not broken ::)