The most common issue for this is that your Internet IP address changed. You should have two connections in the Sibell app = one for 'Local', while you are on location with the NVR and using Wifi connection on your phone. This will use your LAN IP address. (The one you have in the NVR in Network Settings)

Your second connection should be for 'Away' when you are on a 4G connection. (or using a different Wifi somewhere else) This one will use your Internet IP address, which can change periodically. (unless you have purchased a 'Static' IP from your ISP.
When that happens - get on a computer on the same LAN as the NVR, open a web browser and go to http://www.canyouseeme.org. This site will reflect your current Internet IP address - compare that to what you have in your phone app. If it has changed. then justy modify the address in the SiBell mobile app to match the new address.

Let us know if that helps.

Ted Case
Eline / Techpro Support