Tracar client 5.16 on Android 5.1 Send failed

I'm trying to track an Android phone without a SIM. Have WiFi and the client is able to update location but gives send failed error.

Tracar client version is 5.16 & Android version is 5.1

From the browser I can connect to the server which is a custom built one with own domain name.

Can anyone plz help me to find what I'm doing wrong to have the send failed error.

Have other devices and they are working but they have been registered a year ago.

Thanks in advance

Error can mean one of two things:

1. App is not able to connect to the server.
2. Server doesn't reply with success code.

Thanks Anton

Chrome is able to connect to the server from the Android device.
Server is tracking other devices.

How can I further troubleshoot the problem with this device?

You need to send same request to the server and see what the reply is.

Thanks Anton
I've entered http://demo.traccar.org as the tracking server address.
Still it is giving send failed error. Am I entering the correct server address?
Where do I configure port from the client?

Port is part of the URL:

https://en.wikipedia.org/wiki/URL