U2421E, stops working on MacBook Pro, partially solved

I'm posting this in case others run into the same issue I did, so they know how to solve it.

Activating MST via the monitor menu or plugging into the DisplayPort output prevents a U2421E from recognizing USB-C or DisplayPort inputs until a factory reset is performed. To perform a factory reset, you have to connect via HDMI to access the monitor menu.

I got a new Dell U2421E monitor last week. It worked when I initially plugged it in to test. I then proceeded to finish adding it into my setup, which included connected it via DisplayPort to daisy chain it to a second monitor (a Dell U2412M). When I tried turning it on afterwards, it stopped working.

After 2 hours of trying different things, I was able to determine that there is a bug in the firmware for the U2421E monitor that causes it to stop recognizing either USB-C or DisplayPort input when the MST multi-stream feature is enabled. This occurs either when you specifically activate MST using the monitor menu OR when you plug in a DisplayPort cable into the output / daisy chain port.

Once this mode is activated, the monitor no longer recognizes input from either USB-C or DisplayPort from a MacBook Pro 16" (possibly from other devices, but that's the only one I tested with).

To get the monitor to recognize input again, you need to connect an HDMI input source, so you can access the main monitor menu, the reset the monitor to factory settings.

Unfortunately, the main monitor menu is not accessible until the monitor is displaying an input. Until then, it only shows the input connection menu. This is a problem that should be fixed by Dell, since it prevents accessing features unrelated to the input source, such as the factory reset. And it makes solving this issue harder, since you need to connect to HDMI to fix a problem with a USB-C or DisplayPort connection.