Roman Khudobei

I don't know if editing armbian_firstlogin is the right thing to do. Wouldn't it prevent the source code from being updated (git pull) or make it more difficult? Did you already looked at the following links?
https://docs.armbian.com/Developer-Guide_User-Configurations/ https://docs.armbian.com/Developer-Guide_Extensions/ https://docs.armbian.com/Developer-Guide_Extensions-Hooks/ Don't have experience with such things yet, but hope it helps.

so, in case someone needs this too:
General guide to succesfuly use not so popular screen resolutions.
0) Make sure your monitor works with another PCs, and, maybe, with other distros
1) Get your monitor EDID any convinient way - through console ( cat /sys/class/drm/cardX-HDMI-A-Y/edid  X and Y are numbers depending on your setup) or another PC and check if it is OK.
2) Check what clock frequency is used in your EDID, (e.g. min was 71.33 MHz) and pick the closet in from given list: 27, 36, 40, 54, 65, 66, 74.25, 83.5, 108, 106.5 146.25, 148.5.( so i picked 74,25) and use modeline calculator using resolution and clock frequency as only inputs.
3a) EASY WAY but screen gets normal only when DM starts:
put your modeline to  /etc/X11/xorg.conf.d/40-monitor.conf like this:
Section "Monitor"
Identifier "HDMI-X"
Modeline "YOUR_MODE_NAME" 74.25 1280 1290 1300 1500 800 805 810 823 +hsync -vsync //here should be modeline you get from calculator
Option "PreferredMode" "YOUR_MODE_NAME"
EndSection
3b) LONG WAY but you'll be able to see boot messages and use cli interface on 1st login(if you do this before 1st launch of the image)
build your image with CONFIG_DRM_LOAD_EDID_FIRMWARE=y (in config menu search for EDID and it wiil whos where it is) something like device drivers -> graphics support->Direct Render...->enable loading edid insteadof probingfor it
compile custom EDID using modeline from step 2.
edit /boot/armbianEnv.txt: append extraargs=drm.edid_firmware=edid/YOUR_EDID.bin
place YOUR_EDID.bin to /lib/firmware/edid/
 
you're good to go
 

I am working with Rock Pi 4B+ and using image for Rock Pi 4B. Everything works like a charm. Thanks to Armbian team.

I was able to fix the issue with the help of this topic, in case anyone searching

I am also experiencing problems surfing Armbian website here. I tried to look for Rock Pi 4 B images and also for the archive. It also redirected me to "mirror.yandex.ru". Today I could access these pages, because I was redirected to  another source.