Can't setup SmartThings integration?

I have HA set up on Docker (Container Manager) on my Synology NAS. I am using Synology’s built-in DDNS and Reverse Proxy with a Lets Encrypt SSL wildcard certificate.

When I try to install SmartThings integration, I get to this screen:

If I go to the callback URL on that page, I get an ERR_INVALID_RESPONSE.

If I continue on anyways, and input my personal access token, I get the following error:

Logger: homeassistant.components.smartthings.config_flow
Source: components/smartthings/config_flow.py:138
integration: SmartThings (documentation, issues)
First occurred: 6:52:36 AM (1 occurrences)
Last logged: 6:52:36 AM

API error setting up the SmartApp: {'requestId': '9037093691100650629', 'error': {'code': 'ConstraintViolationError', 'message': 'The request is malformed.', 'details': [{'code': 'TargetTimeoutError', 'target': 'https://---------.myds.me/api/webhook/------------------------------------', 'message': 'Upstream target timed out', 'details': []}]}}
Traceback (most recent call last):
  File "/usr/local/lib/python3.13/site-packages/homeassistant/components/smartthings/config_flow.py", line 138, in async_step_pat
    app, client = await create_app(self.hass, self.api)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/homeassistant/components/smartthings/smartapp.py", line 146, in create_app
    app, client = await api.create_app(app)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/pysmartthings/smartthings.py", line 110, in create_app
    entity = await self._service.create_app(app.to_data())
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/pysmartthings/api.py", line 171, in create_app
    return await self.post(API_APPS, data)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/pysmartthings/api.py", line 395, in post
    return await self.request("post", self._api_base + resource, data=data)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/pysmartthings/api.py", line 368, in request
    raise APIResponseError(
    ...<6 lines>...
    )
pysmartthings.errors.APIResponseError: Unprocessable Entity (422): {"requestId": "9037093691100650629", "error": {"code": "ConstraintViolationError", "message": "The request is malformed.", "details": [{"code": "TargetTimeoutError", "target": "https://-----------.myds.me/api/webhook/---------------------------", "message": "Upstream target timed out", "details": []}]}}

What am I doing wrong here?

I figured it out! SmartThings does not support TLS 1.3 (modern).
So I had to lower the TLS level in DSM to 1.3/1.2 (intermediate).