Hmm that's strange. I recently reflashed my own S20 fe 5g stock ROM and everything (excluding Knox related apps such the Samsung health) including ota updates and play certificate seems fine including my own 128gb 8gb ram storage. I have installed it from Sammobile recently
Have you downloaded the correct firmware? I would suggest doing the following:
1. Install your s20 fe 5g ROM along with its correct CSC (or any CSC that you would like). I recommend installing them from Sammobile. No worries if it takes long to install, just be patient
1.5 To flash the firmware, you would need to install Odin on your pc before doing so.
2. Go onto download mode (Connect Cable to pc + volume down + volume up)
3. Open Odin and connect your device onto the PC using your transfer cable
4.You should have 5 slots of opinions. Put the correct files (except user_data and home_csc) onto the correct slots (bl, ap, CP, csc_omc) . Make sure to check F.reset time and auto reboot are toggled on
5. Flash the files using Odin. It would take a while until it's completed. After its completed the device should reboot and factory reset itself, then the device should be working again
6. if you have unlocked your bootloader before make sure to relock it using download mode. this will again factory reset your device but will bring back some features like play certificate and ota updates. Just remember all Knox related apps are unavailable to be restored as they are still impacted even if you reflashed the original firmware. (However Knox features will work if you havent flashed a custom ROM or recovery before so)
7. Uncheck OEM unlock to toggle back all the security features. Remember it won't toggle on the Knox apps as they were already tripped before. (unless you haven't installed a custom ROM or recovery before so)
After that make sure to check your firmware and see if everything goes well. I hope I have helped you.