RP4 dhcpcd.conf file gone after running install script, cant set static IP

RP4 connected to router via LAN. I installed latest 2020-08-20-raspios-buster-armhf.img, updated it, then I set up static IP address in /etc/dhcpcd.conf and all was working ok. RP4 had same IP address after every reboot.

Then I ran OMV5 install script: wget -O - https://github.com/OpenMediaVault-Plugin-Developers....... and install completed ok. But after RP4 restart, I lost my static IP. Also network icon has now two red X on it and mouse over brings: "Connection to dhcpcd lost". I can access to the internet, but my IP address on RP4 is not static any more. It changes at reboot. I tried several times, flashed SD card few times (image checksum verified), ran install script few times, it is always the same. After last try I noticed that dhcpcd.conf is missing after running OMV5 install script. So I manually added it back into /etc/ folder with static settings that worked before, but looks like it is being ignored now, since RP's ip adress is still changing at reboot.

Question, I do need static IP address on my RP4, correct? (I would like to avoid messing with the router if at all possible)

How can I solve this problem? Is this bug in OMV5? Any suggestions?

p.s.

Trying to set up NAS on raspberry Pi for my Linux MX.

Doing this for the first time, I'm not very technical person and I have limited Linux knowledge, so please go easy on me

I have watched several OMV install videos, but didn't find a solution to my problem.

OK will do, thank you. Question, after first boot, do I change dhcpcd.conf to set static IP, before start running OMV5 install script, correct?

Problem is, I'm planning to use RP4 for few other things like running few Python scripts for scraping data from internet etc... I'm Linux beginner and I'm lost without GUI, mouse, desktop...