Your information is probably wrong. Please read pages 19- 22 in the Technical Product Specification document. Additional information you may find in Multiple Display Configuration Options for Intel® NUC Kits NUC8i7HNK and NUC8i7HVK
When i try to set 3840x2160 resolution with 60hz monitor is black and i need to restart nuc.
Please read here https://www.techpowerup.com/gpu-specs/radeon-rx-vega-m-gh.c3056
I have c49hg90 monitor
That TechPowerup article has multiple mistakes in it. I would add that this is describing the discrete device and not the version that is included in the Intel Core i7-8809G processor -- which *is* "designed to have monitors connected to it."
I have simultaneously connected two 4K monitors - both at 60Hz - to the HV NUC and used them without issue.
1. Try to install driver for your monitor.
2. In order to get better information about your system, please download the Intel System Support Utility (SSU). Run the SSU scan and save the results. The .txt file with the results please attach to your post.