mongoDB service error after changing port

Hi, where can i find the logfile you mention ?
Also, i literally fresh created a DigitalOcean server for 3 Days with mongoDB and its hacked. During that time my port was default port and bindIp. So is mongoDB really that easy to get hacked ? What should I do as i really can’t find any reliable up-to-date tutorial online that show up how to set up a proper mongoDB ourself. Thanks