diff --git a/aioshelly/block_device/device.py b/aioshelly/block_device/device.py
index 41fdcdf9..4fca5dbc 100644
--- a/aioshelly/block_device/device.py
+++ b/aioshelly/block_device/device.py
@@ -18,6 +18,7 @@
 from ..common import ConnectionOptions, IpOrOptionsType, get_info, process_ip_or_options
 from ..const import CONNECT_ERRORS, DEVICE_IO_TIMEOUT, HTTP_CALL_TIMEOUT, MODEL_RGBW2
 from ..exceptions import (
+    CustomPortNotSupported,
     DeviceConnectionError,
     FirmwareUnsupported,
     InvalidAuthError,
@@ -118,6 +119,10 @@ async def initialize(self, async_init: bool = False) -> None:
         if self._initializing:
             raise RuntimeError("Already initializing")
 
+        # GEN1 cannot be configured behind a range extender as CoAP port cannot be natted
+        if self.options.port != 80:
+            raise CustomPortNotSupported
+
         self._initializing = True
         self.initialized = False
         ip = self.options.ip_address
diff --git a/aioshelly/exceptions.py b/aioshelly/exceptions.py
index b14f476f..36f6b12d 100644
--- a/aioshelly/exceptions.py
+++ b/aioshelly/exceptions.py
@@ -47,6 +47,10 @@ class MacAddressMismatchError(ShellyError):
     """Raised if input MAC address does not match the device MAC address."""
 
 
+class CustomPortNotSupported(ShellyError):
+    """Raise if GEN1 devices are access with custom port."""
+
+
 class RpcCallError(ShellyError):
     """Raised to indicate errors in RPC call."""
 
diff --git a/tools/example.py b/tools/example.py
index 49030bb0..ae3fb800 100644
--- a/tools/example.py
+++ b/tools/example.py
@@ -26,6 +26,7 @@
 from aioshelly.common import ConnectionOptions
 from aioshelly.const import WS_API_URL
 from aioshelly.exceptions import (
+    CustomPortNotSupported,
     DeviceConnectionError,
     FirmwareUnsupported,
     InvalidAuthError,
@@ -56,6 +57,9 @@ async def test_single(options: ConnectionOptions, init: bool, gen: int | None) -
         except WrongShellyGen:
             print(f"Wrong Shelly generation {gen}, device gen: {2 if gen==1 else 1}")
             return
+        except CustomPortNotSupported:
+            print(f"Custom port ({options.port}) not supported for Gen1")
+            return
 
         print_device(device)