IMac 2010 High Sierra 2nd Screen Monitor HELP

[MrM]

Hi, Thank you for the help today. I have a iMac 2010 running High Sierra (10.13.6) and trying to use it as a second display. I have a Lenovo T480 that is connected Usb-c to Usb-c to a Lenovo Thinkpad USB C Dock Gen 2. I have then connected the 2nd Monitor via Mini Displayport to mini Displayport which is connected to an Female Mini Displayport adapter to Male Displayport that then connects to the Lenovo Thinkpad USB C Dock Gen 2. Every time I press command-F2 the iMac screen goes black for a few seconds then goes back to iMac High Sierra and not me able to use it as a second screen. Am I doing something wrong? Should I not use an adapter? Appreciate any help with this.