Can't connect external display to MacBook Pro

Look at:

 menu > about this Mac > (system report) > Graphics & Displays

When your display is recognized (i.e., answers back over EDID), it will show up there with its name and capabilities, even if somehow it got set to a resolution out-if-range.

HDMI was invented for HD TV sets. It is comparatively difficult to get the appropriate HDMI cabling for displays over HD (1920 by 1080), and it often involves quite a bit of black magic to get 4K displays to work over HDMI.

DisplayPort (or its cousins ThunderBolt and USB-C display) are superior in almost every way, including generating far less waste heat.

to get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, the display will not be shown as present, and no data will be sent to the display.

This query is only sent at certain times:

• at startup

• at wake from sleep

• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go

• on invoking Option-(Detect Display) button in Displays preferences (from another display)

so try doing some of those things and see if the display comes alive.

Modern Displays with multiple ports are sometimes busy scanning the other ports, looking for an input, and miss the query from the Mac. They need to pay attention to the port you are actually using, or they will miss the query.

Some displays have On-Screen Display settings that can be used to tell the display a computer is attached on a certain port, or a certain port should be highest priority. Changing those may make your display more responsive.

Some displays include their own private "sleep" settings for the display alone. This can allow the display to enter its own sleep mode, on top of the Mac's not sending it data. A display that is sleeping on its own cannot respond to the Mac's query, and will stay dark.

Some USB-C cables are power only. If it is the cable that shipped with your power brick, be sure to get an appropriate USB cable that can manage video. Also Some USB-C hubs only get sufficient power from the computer for data. SIIG and OWC both offer excellent hubs that offer 100W throughput and Displayport and HDMI options.

Most monitors use HDMI connections. A simple USB-C to HDMI connection should work fine.

Another thing to consider, the laptop you have may not have enough direct power to manage your display the other laptop does. Make sure that's not the case.