To download and install kernel deb files (standard L4T + the patch to add support for 2.5K and 4K displays) copy and paste these lines to the terminal:
cd /tmp/ && \
wget http://Dragon.Studio/2021/01/nvidia-l4t-kernel-4k-support.zip && \
unzip nvidia-l4t-kernel-4k-support.zip && \
sudo apt install ./*.deb
Please note that this is my first time sharing deb files so if somebody tries them please let me know if they work for you and what Jetson Nano (2GB, B01 or A02) and JetPack you were using. It would be useful to have at least one feedback for each platform so others would know if these packages work for Jetson Nano A02 / B01 / 2GB. I just tested my packages with freshly installed system on Jetson Nano B01 with JetPack 4.5 so they should work at least on this platform.
I soon plan to share a short tutorial about how to build your own kernel .deb, since I was not able to find any tutorials like that for Jetson Nano.
If after finishing booting to the new kernel you still have a black screen, try replugging display power. In my case 2.5K display is USB powered and I had to run USB power cycle tool from USB Power control - #7 by DaneLLL from ~/.xsessionrc. This is not an issue with Jetson Nano but with some badly made HDMI controllers. If after startup you can see an image right away then your display does not have this issue.