Thank you, stanislawrogasik, finally got working 5G and VoWifi on my Pixel 4a (5G) in Slovakia - carrier O2 / Radost!
for anyone else trying, here is what I did in summary - I had to unlock bootloader (command fastboot flashing unlock) and then patch ROM with Magisk and flash with full wipe.
backup everything! you can brick your phone - have a backup phone
download Pixelflasher, install original usb drivers from google, download original rom from google, in my case bramble-t3b3.230413.006-factory-17d94e87.zip , platform-tools_r33.0.3-windows (there are issues with v 34),
enable developer mode in android by tapping buld number multiple times in "about phone" settings, and enable usb debugging, oem unlocking in developer settings under system settings
first unlock the bootloader in fastboot mode with the command from platform tools - fastboot flashing unlock, connect phone via usb, and allow computer in usb debugging on the screen. then in pixelflasher scan and select the device, load the ROM - phone factory image, process it and patch it (adds magisk to rom), select the patched rom - boot.img, select wipe all data and flash pixel phone, wait
after installing the rom and first setup, open Magisk - it will require reboot on first startup - after reboot, in Magisk - Modules, install downloaded ZIP from github - stanislawrogasik/Pixel5-VoLTE-VoWiFi, after reboot 5G should be working and enable wifi calling in sim settings
your bootloader is now unlocked which is less secure and contactless payments do not work out of the box, some banking apps have issues with rooted phones
I installed module kdrag0n/safetynet-fix to allow contactless payments, in Magisk settings I had to enable Zygisk and enforce denylist, in denylist I put all my mobile banking apps and all android / system / OS apps
do not relock the bootloader if you have patched the rom, only relock with stock rom