Question: MacBook won't detect external display
A bit ago I purchased myself a MacBook Pro 15" (2016).
I purchased an adapter so I could connect it to my external monitor (which functions perfectly). It was a USB-C to DisplayPort adapter, after which I used a DisplayPort cable to connect it to my external UHD monitor (Philips Brilliance 288P).
The MacBook Pro was running Sierra as operating system, and it recognized the external monitor.
After I went through a bit of technical problems, however. Eventually, my 2016 model got replaced by a brand-new 2017 model. This one shipped with High Sierra, and is now running version 10.13.
Since the "update" to High Sierra (which I cannot revert, since it shipped with this OS X), my MacBook Pro 15" (2017) does not recognize my external monitor anymore using the exact same method I used before (USB-C to DisplayPort adapter).
The external monitor does not show up in the "display" section, and when connected and turned on simply goes to sleep. Normally it automatically detects a source and switches to that mode, but I also tried manually switching to DisplayPort as an input, after which I immediately receive the message of no signal being found.
I know for sure that the adapter works properly, as I have two of them which both worked before. Little tricks such as trying all USB-C ports, rebooting and reconnecting everything and booting in safe mode are to no use. Since using this new MacBook Pro with High Sierra on it, it simply acts as if my external monitor does not exist.
Does someone have any clue on what to do?[Re-Titled by Host]
MacBook Pro with Retina display, macOS High Sierra (10.13)