Creating RAID5 over existing data is bad

Irreversibly bad, in fact, if you do like here

I bought a Zyxel Nas540 … plugged my HDD’s with my data in them and created a RAID5 volume. I didn’t know it will change the file format completely. I tried to connect my hdd’s back to my computer to backup the data but all of the hdd’s are damaged. … Is there a way to restore last file system ?

No.

The RAID5 has parity interleaved with data on all disks. Once RAID5 is first synchronized, every Nth block of data on each of N disks is overwritten. Given typical block sizes used in RAIDs, this prevents any practical recovery.