There are a few reasons you may see this notification.
The collar is off
The collar is on but it doesn’t have cellular service.
If your collar has service, you should see white bars like a smartphone to the right of the letters “BT” on your collar display. If you see “NS” your collar has no service.
If you have not subscribed to a cellular plan for your system, ignore the notification and tap “OK”. If you have subscribed to the cellular service, your dog (and collar) is likely in an area with poor AT&T/T-Mobile cell service. As your dog moves around your property it will continue to try connecting to the closest AT&T/T-Mobile cell towers. Note even though SpotOn uses AT&T and T-Mobile 3G networks (not Verizon), it will still communicate with your phone regardless of which cell carrier you use. (Just like you can call your mom even though you use AT&T and she has MetroPCS)