Tailscale's relay servers ... are DHCP settings, Client lists, and NAT/LAN settings. What your looking for is the ability to set a client to have a certain IP whether that be via MAC address or client ...
A modern, scalable, robust DHCPv4 and DHCPv6 server, with database (MySQL, PostgreSQL), hooks, multi-threading, RADIUS, NETCONF, Kerberos and more.