diff --git a/openwisp_controller/config/migrations/0037_alter_taggedtemplate.py b/openwisp_controller/config/migrations/0038_alter_taggedtemplate.py
similarity index 94%
rename from openwisp_controller/config/migrations/0037_alter_taggedtemplate.py
rename to openwisp_controller/config/migrations/0038_alter_taggedtemplate.py
index 93315a247..118396d1d 100644
--- a/openwisp_controller/config/migrations/0037_alter_taggedtemplate.py
+++ b/openwisp_controller/config/migrations/0038_alter_taggedtemplate.py
@@ -8,7 +8,7 @@ class Migration(migrations.Migration):
 
     dependencies = [
         ('contenttypes', '0002_remove_content_type_name'),
-        ('config', '0036_device_group'),
+        ('config', '0037_alter_management_ip_help_text'),
     ]
 
     operations = [
diff --git a/openwisp_controller/config/migrations/0038_vpn_subnet.py b/openwisp_controller/config/migrations/0039_vpn_subnet.py
similarity index 94%
rename from openwisp_controller/config/migrations/0038_vpn_subnet.py
rename to openwisp_controller/config/migrations/0039_vpn_subnet.py
index 171e45c5f..c367040ae 100644
--- a/openwisp_controller/config/migrations/0038_vpn_subnet.py
+++ b/openwisp_controller/config/migrations/0039_vpn_subnet.py
@@ -9,7 +9,7 @@ class Migration(migrations.Migration):
 
     dependencies = [
         migrations.swappable_dependency(settings.OPENWISP_IPAM_SUBNET_MODEL),
-        ('config', '0037_alter_taggedtemplate'),
+        ('config', '0038_alter_taggedtemplate'),
     ]
 
     operations = [
diff --git a/openwisp_controller/config/migrations/0039_wireguard_vxlan_ipam.py b/openwisp_controller/config/migrations/0040_wireguard_vxlan_ipam.py
similarity index 99%
rename from openwisp_controller/config/migrations/0039_wireguard_vxlan_ipam.py
rename to openwisp_controller/config/migrations/0040_wireguard_vxlan_ipam.py
index d770cf957..2648d75e5 100644
--- a/openwisp_controller/config/migrations/0039_wireguard_vxlan_ipam.py
+++ b/openwisp_controller/config/migrations/0040_wireguard_vxlan_ipam.py
@@ -13,7 +13,7 @@ class Migration(migrations.Migration):
         swapper.dependency('pki', 'Ca'),
         swapper.dependency('openwisp_ipam', 'IpAddress'),
         swapper.dependency('openwisp_ipam', 'Subnet'),
-        ('config', '0038_vpn_subnet'),
+        ('config', '0039_vpn_subnet'),
     ]
 
     operations = [
diff --git a/openwisp_controller/config/migrations/0040_vpnclient_ip_setnull.py b/openwisp_controller/config/migrations/0041_vpnclient_ip_setnull.py
similarity index 95%
rename from openwisp_controller/config/migrations/0040_vpnclient_ip_setnull.py
rename to openwisp_controller/config/migrations/0041_vpnclient_ip_setnull.py
index 0f359bd8f..857ddab9a 100644
--- a/openwisp_controller/config/migrations/0040_vpnclient_ip_setnull.py
+++ b/openwisp_controller/config/migrations/0041_vpnclient_ip_setnull.py
@@ -9,7 +9,7 @@ class Migration(migrations.Migration):
 
     dependencies = [
         migrations.swappable_dependency(settings.OPENWISP_IPAM_IPADDRESS_MODEL),
-        ('config', '0039_wireguard_vxlan_ipam'),
+        ('config', '0040_wireguard_vxlan_ipam'),
     ]
 
     operations = [
diff --git a/openwisp_controller/connection/base/models.py b/openwisp_controller/connection/base/models.py
index 3e6afcce2..42c75618a 100644
--- a/openwisp_controller/connection/base/models.py
+++ b/openwisp_controller/connection/base/models.py
@@ -1,6 +1,5 @@
 import collections
 import logging
-import operator
 
 import jsonschema
 from django.core.exceptions import ValidationError
@@ -148,7 +147,6 @@ def auto_add_to_devices(cls, credential_id, organization_id):
             conn = DeviceConnection(
                 device=device, credentials_id=credential_id, enabled=True
             )
-            conn._connector_field = 'credentials.connector'
             conn.full_clean()
             device_connections.append(conn)
             # Send create query when chunk_size is reached
diff --git a/openwisp_controller/connection/connectors/snmp.py b/openwisp_controller/connection/connectors/snmp.py
index 1653cbd3d..eb9ce4a3d 100644
--- a/openwisp_controller/connection/connectors/snmp.py
+++ b/openwisp_controller/connection/connectors/snmp.py
@@ -1,5 +1,4 @@
 import logging
-from io import StringIO
 
 from jsonschema import validate