iMac mid 2011 reboots when connecting an external display

I'm new to apple, and finally got my second hand iMac booted. I try to connect an external display to the iMac with an mini display port adapter. I have 2 adapters, 1 does nothing, the other makes my external display working (FHD and 4K), but then, the iMac reboots.

I think this has something to do with the switching of the graphics adapter. I searched for a workaround to disable the AMD graphics adapter, but can't find this.

Does anyone has another solution, or tips for a work around?

It is safest to treat DisplayPort as not hot-pluggable.

Only connect or disconnect when the computer is off. Power on the display before the computer [it may go straight back to sleep, this is not a problem, it will re-awaken during boot.]

It theoretically should be capable, but relies on the computer, display & cable to all be compliant [& not buggy].
Google "display port hot plug" to see some of the issues it has.