How to properly mount NFSv4 share from Synology NAS to Debian desktop

Enable NFSv4 on Synology, setup your domain. Say, it is “lan”.

On a client machine, set the same domain, enable idmapd in NFS config, forbid disabling of idmapping for kernel modules.

sudo sed -i 's/NEED_IDMAPD=/NEED_IDMAPD=yes/' /etc/default/nfs-common
sudo sed -i 's/# Domain = localdomain/Domain = lan/ /etc/idmapd.conf
echo "
options nfs nfs4_disable_idmapping=0
options nfsd nfs4_disable_idmapping=0
" | sudo tee /etc/modprobe.d/nfs-imapd.conf

In /etc/fstab file, mount your shares as nfs4, add sec=sys to mount options.

synology-nas-ip:/volume1/myshare  /home/username/myshare  nfs4    defaults,sec=sys  0 0

Do not forget to reload kernel modules and NFS-related services, or just restart the client machine before mounting.