Diagnosis
This is almost always a WiFi handshake problem. Your camera is powering up but can't maintain a stable connection to your home network, so the app gives up and shows it as offline.
How to Fix It
First, power cycle the whole system. 1) Unplug the camera's power adapter from the outlet. 2) Wait a full 30 seconds. 3) Plug it back in and give the camera a solid 2-3 minutes to fully reboot and reconnect. Check the app after that. If it's still offline, the signal might be weak. You need to check the camera's reported signal strength in the SimpliSafe app under Device Settings > your camera > About Device. If it's 'Poor' or 'Fair,' that's your culprit.
If That Didn't Work
If the signal is weak, your router might be too far away or there's too much wall/obstruction in between. The permanent fix is to get a WiFi extender that creates a new network closer to the camera's mount. Set the extender up, then re-run the camera's setup in the app to connect it to the extender's network name instead of your main one.
Heads Up
When checking signal, do it from your phone standing right next to the camera's location. Your phone's signal isn't the same as the camera's, but it's a good quick test. If your phone's WiFi is also one bar there, you definitely need an extender.