@Lyssa Seems like your camera can't connect to the cloud server. Under normal circumstances, the camera should be online as long as your phone connecting to the Internet.
Maybe you can try some troubleshooting steps. When your camera is offline on tapo app, tap on it and tap 'detailed', there are some troubleshooting advices. And here I find a FAQ related to your issue (case2), you can check it.
I guess your camera's LED light status might be amber, it means that it connect to the WiFi but can't connect to the cloud server. So that you can't visit it without the same network. In this case, you can try to change your router's DNS to 8.8.8.8 and move your camera close to your router temporarily, then try again in 4G network.