diff --git a/services/router.nix b/services/router.nix
index 1685eb1..fc9ee2c 100644
--- a/services/router.nix
+++ b/services/router.nix
@@ -114,6 +114,13 @@ in
           dhcpV4Config = {
             UseDNS = false;      # don't overwrite resolv.conf from ISP DNS
             UseHostname = false;
+            # networkd defaults SendRelease=true: stopping the unit (every
+            # nixos-upgrade, and the watchdog's 4min escalation) sends a
+            # DHCPRELEASE that hands our IP back to the ISP pool, so the next
+            # start DISCOVERs a fresh one. Every WAN IP change in the journal
+            # landed at 05:1x — the upgrade window — not on ISP timing.
+            # false = keep the lease and re-REQUEST it (INIT-REBOOT).
+            SendRelease = false;
           };
         };
         "20-lan" = {