Provide a more detailed error message than "No known servers"

src/i18n/strings/en_EN.json

@@ -808,6 +808,12 @@
     "Please contact your homeserver administrator.": "Please contact your homeserver administrator.",
     "The person who invited you has already left.": "The person who invited you has already left.",
     "The person who invited you has already left, or their server is offline.": "The person who invited you has already left, or their server is offline.",
+    "This Room ID (%(id)s) cannot be joined. Try to join using a Room Alias (#room:example.com) instead.": "This Room ID (%(id)s) cannot be joined. Try to join using a Room Alias (#room:example.com) instead.",
+    "Why?": "Why?",
+    "A Room ID alone cannot be used to join a room unless your homeserver already participates in that room.": "A Room ID alone cannot be used to join a room unless your homeserver already participates in that room.",
+    "For a Room ID to be joinable over federation, a list of homeservers to attempt to join through must be provided. You didn't provide any.": "For a Room ID to be joinable over federation, a list of homeservers to attempt to join through must be provided. You didn't provide any.",
+    "The sole purpose of the domain at the end of a Room ID is to ensure it is unique.": "The sole purpose of the domain at the end of a Room ID is to ensure it is unique.",
+    "For these reasons, Room Aliases are the recommended way to join rooms.": "For these reasons, Room Aliases are the recommended way to join rooms.",
     "Failed to join": "Failed to join",
     "Connection lost": "Connection lost",
     "You were disconnected from the call. (Error: %(message)s)": "You were disconnected from the call. (Error: %(message)s)",

src/stores/RoomViewStore.tsx

@@ -437,7 +437,7 @@ export class RoomViewStore extends Store<ActionPayload> {
             </div>;
         } else if (err.httpStatus === 404) {
             const invitingUserId = this.getInvitingUserId(roomId);
-            // only provide a better error message for invites
+            // provide a better error message for invites
             if (invitingUserId) {
                 // if the inviting user is on the same HS, there can only be one cause: they left.
                 if (invitingUserId.endsWith(`:${MatrixClientPeg.get().getDomain()}`)) {
@@ -446,6 +446,33 @@ export class RoomViewStore extends Store<ActionPayload> {
                     description = _t("The person who invited you has already left, or their server is offline.");
                 }
             }
+
+            // if joined through room ID and didn't provide any via servers, provide
+            // a more detailed error than "No known servers"
+            if (roomId === this.state.roomId && !this.state.viaServers?.length) {
+                description = <div>
+                    { _t("This Room ID (%(id)s) cannot be joined. Try to join using a Room " +
+                         "Alias (#room:example.com) instead.", {
+                        id: this.state.roomId,
+                    }) }<br /><br />
+
+                    <details>
+                        <summary>{ _t("Why?") }</summary><br />
+
+                        { _t("A Room ID alone cannot be used to join a room unless your " +
+                             "homeserver already participates in that room.") }<br /><br />
+
+                        { _t("For a Room ID to be joinable over federation, a list of " +
+                             "homeservers to attempt to join through must be provided. " +
+                             "You didn't provide any.") }<br /><br />
+
+                        { _t("The sole purpose of the domain at the end of a Room ID " +
+                             "is to ensure it is unique.") }<br /><br />
+
+                        { _t("For these reasons, Room Aliases are the recommended way to join rooms.") }
+                    </details>
+                </div>;
+            }
         }
 
         Modal.createDialog(ErrorDialog, {