This. Go watch this: https://www.youtube.com/watch?v=c4IHrKWzxiI
Tl;DR;
1. Enable SSH
2. Run those commands, stop and start tailscale APP in DSM.
sudo setcap cap_net_admin+eip /var/packages/Tailscale/target/bin/tailscaled
• sudo mkdir -p /dev/net
• sudo chmod 0755 /dev/net
• sudo mknod /dev/net/tun c 10 200
• sudo chmod 0666 /dev/net/tun