Hi @cervenka,
I just wrote about upgrading from 19.07 to 21.02 here: Update for UBNT-ERX from 19.07 to 21.02 - #2 by rogerpueyo
Once OpenWrt is installed, the sysupgrade image should be used to upgrade the system (e.g., to a newer version), and not the initramfs nor the factory ones. The sysupgrade image is built specifically for upgrading the system, keeping previous settings over the process if needed, and leaving it operational. Instead, the initramfs image is to be used as a temporal OpenWrt "tool" image that you can boot and, from there, install the sysupgrade image.
If you're lucky enough, you'll be able to enter in OpenWrt's recovery mode without needing the serial port. From there you should flash the sysupgrade image. Otherwise, you'll need to recover your device using the TFTP method.
Best,