The port scanner shows some open ports even though I'm sure they are closed

This typically happens when there's some proxy/firewall between you and the scanned device. For instance, some cell network providers proxy all network requests, so all ports appear open. In this case, when the port scanner tries to connect to the scanned device, the connection attempt is captured by the proxy, so in fact you are connecting to the proxy and not to the remote device (the proxy then connects to the scanned device on your behalf and forwards the data to you).

Also make sure you know where you are performing the port scan from. For instance, if you do a port scan of your router to make sure no ports are open from the Internet, you should get your external IP, disconnect from your wifi, and perform the port scan of the external IP. If you do this while connected to your wifi, you will find open ports for, e.g., the web interface (80, 443) and possibly other services, which are, however, only visible from your LAN and not from the Internet.