Cant get MAS work in element X ios

[Sinterdial]

Describe the bug
I've already set up my self-hosted Synapse and MAS, and everything works fine on the web and desktop clients — they can correctly redirect to the MAS website.
However, when I try to log in to my self-hosted server using Element X on iPhone or Android, it fails to redirect properly and shows the message:
“Your homeserver needs to be upgraded to support Matrix Authentication Service and account creation.”

What’s special about my setup is that my Synapse instance isn’t running on a standard port — meaning my homeserver name and various URIs all include the domain name plus a port number. I’m not sure if that’s what’s causing the issue.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'Element X'
2. Click on 'Create Account'
3. Chose 'Change account provider'
4. Insert my synapse server url
5. Click on 'Continue'
6. See error**“Your homeserver needs to be upgraded to support Matrix Authentication Service and account creation.”**

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: [Windows 11 24H2]
• Browser [Selfhosted element-web ]
• Version [v1.12.0]

Smartphone (please complete the following information):

• Device: [iPhone 16 Pro]
• OS: [iOS 26]
• Browser [chrome]
• Version [25.10.0]

Additional context
My setup environment: Docker (Portainer CE)
The Synapse container reports an error:

2025-10-18 15:08:40,145 - synapse.http.client - 454 - INFO - GET-413 - Received response to GET http://matrix-auth-service:8094/.well-known/openid-configuration: 502
2025-10-18 15:08:39,968 - synapse.access.http.8008 - 515 - INFO - GET-412 - 192.168.5.25 - 8008 - {None} Processed request: 0.000sec/0.000sec (0.000sec, 0.000sec) (0.000sec/0.000sec/0) 49B 200 "GET /_matrix/federation/v1/version HTTP/1.1" "Element X/25.10.0 (iPhone 16 Pro; iOS 26.0.1; Scale/3.00)" [0 dbevts]
2025-10-18 15:08:40,145 - synapse.http.server - 151 - ERROR - GET-413 - Failed handle request via 'AuthMetadataServlet': <XForwardedForRequest at 0x14cb3bb5c920 method='GET' uri='/_matrix/client/unstable/org.matrix.msc2965/auth_metadata' clientproto='HTTP/1.1' site='8008'>
    callback_return = await self._async_render(request)
Traceback (most recent call last):
    raise self.value.with_traceback(self.tb)
2025-10-18 15:08:40,145 - synapse.http.client - 454 - INFO - GET-413 - Received response to GET http://matrix-auth-service:8094/.well-known/openid-configuration: 502
  File "/usr/local/lib/python3.12/site-packages/synapse/http/server.py", line 339, in _async_render_wrapper
  File "/usr/local/lib/python3.12/site-packages/synapse/http/server.py", line 564, in _async_render
  File "/usr/local/lib/python3.12/site-packages/synapse/rest/client/auth_metadata.py", line 99, in on_GET
    callback_return = await raw_callback_return
                      ^^^^^^^^^^^^^^^^^^^^^^^^^
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    return 200, await self._auth.auth_metadata()
    metadata = await self._server_metadata.get()
  File "/usr/local/lib/python3.12/site-packages/synapse/api/auth/mas.py", line 172, in auth_metadata
  File "/usr/local/lib/python3.12/site-packages/synapse/util/caches/cached_call.py", line 143, in get
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/synapse/util/caches/cached_call.py", line 113, in get
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    return await self._cachedcall.get()
    self._result.raiseException()
  File "/usr/local/lib/python3.12/site-packages/twisted/python/failure.py", line 455, in raiseException
  File "/usr/local/lib/python3.12/site-packages/twisted/internet/defer.py", line 1857, in _inlineCallbacks
    result = context.run(gen.send, result)
  File "/usr/local/lib/python3.12/site-packages/synapse/util/caches/cached_call.py", line 133, in _wrapper
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
           ^^^^^^^^^
    return await f()
    response = await self._http_client.get_json(self._metadata_url)
  File "/usr/local/lib/python3.12/site-packages/synapse/api/auth/mas.py", line 159, in _load_metadata
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    body = await self.get_raw(uri, args, headers=actual_headers)
  File "/usr/local/lib/python3.12/site-packages/synapse/http/client.py", line 603, in get_json
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/synapse/http/client.py", line 695, in get_raw
    raise HttpResponseException(
synapse.api.errors.HttpResponseException: 502: Bad Gateway

The log in MAS container:

2025-10-18T07:08:40.155373Z  INFO http.server.response GET-946 - "GET /_matrix/client/v3/login HTTP/1.1" 200 OK "Element X/25.10.0 (iPhone 16 Pro; iOS 26.0.1; Scale/3.00)" [polls: 0, cpu: 0.0ms, elapsed: 0.0ms]
2025-10-18T07:08:16.788200Z  INFO http.server.response GET-920 - "GET /_matrix/client/v3/login HTTP/1.1" 200 OK "Element X/25.10.0 (vivo V2338A; Android 15; PD2338_A_15.1.13.15.W10.V000L1; Sdk c7b4b5dc0)" [polls: 0, cpu: 0.0ms, elapsed: 0.1ms]
2025-10-18T07:08:56.876982Z  INFO mas_tasks::new_queue:507 worker-run-loop-962 - Sending heartbeat
2025-10-18T07:09:40.907702Z  INFO http.server.response POST-1010 - "POST /oauth2/introspect HTTP/1.1" 200 OK "Synapse/1.139.2" [polls: 7, cpu: 0.3ms, elapsed: 0.9ms]
2025-10-18T07:09:40.903535Z  INFO http.server.response POST-1009 - "POST /oauth2/introspect HTTP/1.1" 200 OK "Synapse/1.139.2" [polls: 7, cpu: 0.3ms, elapsed: 2.8ms]
2025-10-18T07:09:48.176450Z  INFO mas_handlers::activity_tracker::worker:242 Flushing 2 activity records to the database