When you add an SMB/CIFS share via dietpi-drive_manager, it loops through all existing SMB protocol versions from the highest 3.1.1 to the lowest 1.0 and tries to connect with all of them, before adding the first functional one to /etc/fstab.
If you use the mount source //192.168.5.1/dietpi, this means there needs to exist an SMB share called “dietpi” on the server. Given it is a DrayTek server and no DietPi system, is this the case?
The version is set via the vers=n.m mount option, which you used already. Check out the man pages as well about other mount options and possible hints: https://manpages.debian.org/mount.cifs