diff --git a/api/ddns_manager.py b/api/ddns_manager.py
index f47aba2..8b508bc 100644
--- a/api/ddns_manager.py
+++ b/api/ddns_manager.py
@@ -386,27 +386,29 @@ class DDNSManager(BaseServiceManager):
     def register(self, name: str, ip: str) -> dict:
         """Register the cell's subdomain with the configured provider.
 
-        Stores the returned token in the identity config under
-        identity['domain']['ddns']['token'] and records the subdomain.
+        Fetches the public IP via ipify when ip is empty.
+        Stores the returned token in the top-level ddns config (where
+        update_ip reads it) and updates _identity.domain_name.
         Returns the dict from provider.register().
         """
         provider = self.get_provider()
         if provider is None:
             raise DDNSError("No DDNS provider configured")
 
+        if not ip:
+            ip = _get_public_ip() or ''
+
         result = provider.register(name, ip)
 
-        # Persist token + subdomain back into identity
-        identity = self._identity()
-        domain_cfg = dict(identity.get('domain', {}))
-        ddns_cfg = dict(domain_cfg.get('ddns', {}))
-        if 'token' in result:
-            ddns_cfg['token'] = result['token']
-        if 'subdomain' in result:
-            ddns_cfg['subdomain'] = result['subdomain']
-        domain_cfg['ddns'] = ddns_cfg
         if self.config_manager is not None:
-            self.config_manager.set_identity_field('domain', domain_cfg)
+            # Token lives in the top-level ddns config so update_ip() can find it
+            if 'token' in result:
+                ddns_cfg = dict(self.config_manager.configs.get('ddns', {}))
+                ddns_cfg['token'] = result['token']
+                self.config_manager.set_ddns_config(ddns_cfg)
+            # Keep domain_name in identity up to date
+            if 'subdomain' in result:
+                self.config_manager.set_identity_field('domain_name', result['subdomain'])
 
         self._last_ip = ip
         return result
diff --git a/tests/test_ddns_manager.py b/tests/test_ddns_manager.py
index 430d14a..43c8d08 100644
--- a/tests/test_ddns_manager.py
+++ b/tests/test_ddns_manager.py
@@ -307,22 +307,50 @@ class TestUpdateIp(unittest.TestCase):
 # ---------------------------------------------------------------------------
 
 class TestRegister(unittest.TestCase):
-    def test_register_stores_token_in_config(self):
+    def test_register_stores_token_in_ddns_config(self):
         cm = _make_config_manager(ddns_cfg={'provider': 'pic_ngo'})
         mgr = DDNSManager(config_manager=cm)
 
         mock_provider = MagicMock()
-        mock_provider.register.return_value = {'token': 'new_tok', 'subdomain': 'alpha'}
+        mock_provider.register.return_value = {'token': 'new_tok', 'subdomain': 'alpha.pic.ngo'}
         mgr.get_provider = MagicMock(return_value=mock_provider)
 
         result = mgr.register('alpha', '1.2.3.4')
         self.assertEqual(result['token'], 'new_tok')
 
-        # set_identity_field('domain', ...) should have been called
-        cm.set_identity_field.assert_called_once()
-        field_name, field_value = cm.set_identity_field.call_args[0]
-        self.assertEqual(field_name, 'domain')
-        self.assertEqual(field_value['ddns']['token'], 'new_tok')
+        # Token saved to top-level ddns config so update_ip() can find it
+        cm.set_ddns_config.assert_called_once()
+        saved_ddns = cm.set_ddns_config.call_args[0][0]
+        self.assertEqual(saved_ddns['token'], 'new_tok')
+
+        # Subdomain saved to _identity.domain_name
+        cm.set_identity_field.assert_called_once_with('domain_name', 'alpha.pic.ngo')
+
+    def test_register_fetches_public_ip_when_empty(self):
+        cm = _make_config_manager(ddns_cfg={'provider': 'pic_ngo'})
+        mgr = DDNSManager(config_manager=cm)
+
+        mock_provider = MagicMock()
+        mock_provider.register.return_value = {'token': 't', 'subdomain': 'alpha.pic.ngo'}
+        mgr.get_provider = MagicMock(return_value=mock_provider)
+
+        with patch('ddns_manager._get_public_ip', return_value='5.6.7.8') as mock_ip:
+            mgr.register('alpha', '')
+        mock_ip.assert_called_once()
+        mock_provider.register.assert_called_once_with('alpha', '5.6.7.8')
+
+    def test_register_uses_provided_ip_without_fetching(self):
+        cm = _make_config_manager(ddns_cfg={'provider': 'pic_ngo'})
+        mgr = DDNSManager(config_manager=cm)
+
+        mock_provider = MagicMock()
+        mock_provider.register.return_value = {'token': 't', 'subdomain': 'alpha.pic.ngo'}
+        mgr.get_provider = MagicMock(return_value=mock_provider)
+
+        with patch('ddns_manager._get_public_ip') as mock_ip:
+            mgr.register('alpha', '1.2.3.4')
+        mock_ip.assert_not_called()
+        mock_provider.register.assert_called_once_with('alpha', '1.2.3.4')
 
     def test_register_raises_when_no_provider(self):
         cm = _make_config_manager()