There were three options of video hardware for that model with 1GB, 2GB, and 4GB of video RAM (VRAM); what yours has can affect what resolutions you can run externally.
Please do "About this Mac" from your Apple menu. It will appear in the middle of the black stripe but you can drag that window to a clear area. That window shows your video hardware type and VRAM amount where I put the "video hardware" label in this screenshot:
That can help others better advise you.
The Apple Tech Specs for that model shows this:
- Simultaneously supports full native resolution on the built-in display and up to a 30-inch display (2560 by 1600 pixels) on an external display
but does not specify which of those three video chipsets was used. I suspect that is the entry-level 1GB chipset.
However, the MacTracker database shows "Up to 3840 by 2160 pixels at 60Hz." As good as MacTracker is, I tend to go with factory recommendations, unless you find you have more than the base video chipset.
⚠️ BEFORE YOUR BUY ANYTHING: A video defect can be due the display panel and cabling itself OR due to the video chipset on the logic board. If the display is the issue, your fix should work. If the logic board's video chipset has failed, your fix will be useless. As the video hardware is not slotted but rather integral with the logic board, the only fix for bad video hardware is a new logic board.
So...I strongly recommend finding any compatible external monitor--beg, borrow, but don't steal--and making certain that you get a proper image on an external monitor before buying a big fancy model.
A big solid stripe can be indicative of a failed display panel, but not always.
Use external monitors with your Mac - Apple Support
