--- a/synapse/rest/well_known.py	2025-02-10 21:14:15.979305321 +0000
+++ b/synapse/rest/well_known.py	2025-02-10 21:15:09.627557944 +0000
@@ -107,7 +107,7 @@
         super().__init__()
         self._serve_server_wellknown = hs.config.server.serve_server_wellknown
 
-        host, port = parse_server_name(hs.config.server.server_name)
+        host, port = parse_server_name(hs.config.server.public_baseurl.rstrip('/').split("://")[1])
 
         # If we've got this far, then https://<server_name>/ must route to us, so
         # we just redirect the traffic to port 443 instead of 8448.