I agree with HP - an incompatible resolution could certainly cause this problem, although it could be something else.
The recommended resolution for the HP Pavilion 22cwa 21.5-inch IPS LED Backlit Monitor you purchased is 1920 x 1080 pixels, so you could certainly play around with your settings in Apple > System Preferences > Displays to see what impact they have.
The other thing you can check are the various settings on the monitor itself by pressing the Menu button to access the On-Screen Display (OSD). One option that I've seen help many users is changing the Input settings on their monitors, such as turning the automatic input setting off and manually selecting the input.
If all of the above fails, there are some troubleshooting steps you can try on your MBP.
Reset the NVRAM
Older Macs had what's called Parameter RAM (PRAM), newer Macs use Non-Volatile Random-Access Memory (NVRAM). To reset this on your particular MBP, follow these steps:
- Shut down your machine. Yes, a full shut down, not just logging out.
- Press the power button and then press the commandoptionpr keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
- Hold those keys down until your Mac reboots again (i.e. you hear the startup chime a 2nd time).
- Let go of the keys and let your Mac reboot normally.
Note: When you log back in you may need to readjust some of your system preferences (e.g. speaker volume, screen resolution, startup disk selection, time zone information, etc).
Boot into Safe Mode
Follow these steps to boot your Mac into Safe Mode:
- Fully shut down your Mac
- Restart your Mac
- Immediately press the Shift key and keep it down
- Let go of the Shift key when you see the login window (NOTE: If you have FileVault enabled you may need to log in twice).
- Take a note of what happens (i.e. can you connect to the monitor okay, etc)
- Restart your Mac as normal to exit Safe Mode
- Test again to see what happens
Once you've booted into Safe Mode, let me know how you went and we'll go from there.
Best Answer
If you want to be able to charge your laptop at the same time, you'll can use a Thunderbolt 3 dock or hub to connect the two monitors at the same time as well as charging through a single port on your laptop - leaving one port available for charging.
A product that will allow you to do this could be the OWC Thunderbolt 3 dock:
https://www.owcdigital.com/products/thunderbolt-3-dock-14-port
You would then connect your existing monitor using your existing cable to the dock. use a mini-Displayport to DisplayPort cable to connect your U2417H monitor to the dock. The dock comes with a Thunderbolt 3 cable that connects the laptop to the dock. The dock provides charging to the laptop.
There exist similar products at various price points from several other brands.
If you want to use your existing dongle you can simply connect your charger to the dongle, connect the dongle to the laptop and use an HDMI-to-HDMI cable to connect the U2417H monitor. The existing monitor uses the same cable as before.
This is obviously much cheaper for you, but you'll need to plug two cables into the laptop leaving no ports free for other uses.