Dell P2719H, No HDMI signal from your device, MBP 2015

I'm trying to connect my MacBook Pro (Retina, 15-inch, Mid 2015) to my new Dell P2719H monitor with an HDMI cable, but I keep getting the "No HDMI signal from your device" message. I've tried turning the monitor/laptop on and off, plugging and unplugging the power/HDMI cables, connecting things in different orders, etc..almost everything - I think. The monitor is definitely reacting to my plugging of the HDMI cable because it blinks after the connection which is then followed by the message, however my MBP doesn't react. I also ran the Built-In Diagnostics test, which it passed. Lastly, I tried connecting the monitor to my co-worker's non-Apple laptop but we had the same issue.

I don't have a Mac Mini DisplayPort adaptor for the DisplayPort cable to monitor came with, so I haven't been able to test that out. The HDMI cable I'm using is brand-new, so I hope that's not the issue. Any suggestions?