Mongoose not connecting after using Port forwarding

Question

I get the Error:

err = new ServerSelectionError();
          ^
MongooseServerSelectionError: connect ECONNREFUSED **My Server IP/ PORT**
    at _handleConnectionErrors (C:\Users\elias\source\repos\Website-Backend\node_modules\mongoose\lib\connection.js:791:11)
    at NativeConnection.openUri (C:\Users\elias\source\repos\Website-Backend\node_modules\mongoose\lib\connection.js:766:11)
    at processTicksAndRejections (node:internal/process/task_queues:95:5) {
  reason: TopologyDescription {
    type: 'Unknown',
    servers: Map(1) { '**My Domain/ PORT**' => [ServerDescription] },
    stale: false,
    compatible: true,
    heartbeatFrequencyMS: 10000,
    localThresholdMS: 15,
    setName: null,
    maxElectionId: null,
    maxSetVersion: null,
    commonWireVersion: 0,
    logicalSessionTimeoutMinutes: null
  },
  code: undefined
}

This makes no sense because I connect with Localhost through Port forwarding MongDB Compass uses the Same Connection String and works perfectly.

mongodb://Username:Password.@localhost:PORT/?authMechanism=DEFAULT

ssh -L **PORT**:localhost:**PORT** **User**@**IP**

I tried allready switching Ports etc. The only Sollution is to open the Port in the Firewall but that is not really practical and secure. I think Mongoose is logging in to then get the IP of the Source and than trying to connect with that IP for some reason?