Hi @mitsos,
It seems that @s_2 has created a very easy way to encrypt the factory file. I have not tested the file itself as I do not own the device. I just downloaded the factory file for the DIR-853-A3 from the openwrt and encrypted it with his tool. You need to be aware that bricking your device will not be recoverable without using a USB TTL.
Before sharing the file I want to share how to use the tool as the download link expires after a while.
This is taken from here
git clone https://github.com/s-2/firmware-utils.git
cd firmware-utils
git checkout dlink-sge-image_2k23
cmake .
make dlink-sge-image
Then use the tool to encrypt the image. Running the tool will give help on how to use it. For the snapshot version these commands can be used.
wget https://downloads.openwrt.org/releases/23.05.0/targets/ramips/mt7621/openwrt-23.05.0-ramips-mt7621-dlink_dir-853-a3-squashfs-factory.bin
./dlink-sge-image DIR-853 openwrt-23.05.0-ramips-mt7621-dlink_dir-853-a3-squashfs-factory.bin openwrt-dir-853-enc.bin
EDIT: Updated for stable 23.05