Your MacBook Pro (Retina, 13-inch, Early 2015) has
- Two Thunderbolt 2 ports with Mini DisplayPort connectors that can double as Mini DisplayPorts
- Two USB-A (USB 3.0) ports
I'm guessing from "mini HDMI" that you're talking about a portable monitor like this one:
https://kooruimonitors.com/products/koorui-15b1-15-6-ips-full-hd-portable-monitor
I've got a cable that connects from the laptop's USB port to the screen's USB-C port but the screen says 'no signal'.
That's because there is none. One thing that USB-C has, that USB-A doesn't, is the ability to carry DisplayPort Alt Mode signals. The Koorui monitor no doubt is looking for DisplayPort Alt Mode input on its USB-C port. But all you are giving it is USB input.
I'm stumped. Any help appreciated.
It sounds like you need a Mini DisplayPort to Mini HDMI connection, from one of the Thunderbolt 2 ports on your MBP to the Mini HDMI port on the Koorui monitor. This will only carry video, not power, so I assume that you will also need to power or charge the Koorui monitor with the aid of a USB-C charger and USB-C cable.
On Amazon, you can find cables with a Mini DisplayPort plug on one end, and a Mini HDMI one on the other.
https://www.amazon.com/s?k=mini+display+port+to+mini+hdmi
