I had to set NFS options in synology to: squash Map root to admin.
I mounted with these options:
nfs4 rw,relatime,vers=4.1,rsize=131072,wsize=131072,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,clientaddr=192.168.1.xxx,local_lock=none,addr=192.168.1.xxx,insecure 0 0