Helium Explorer automatically assumes your Hotspot is Offline because your device has not participated in proof-of-coverage in a while. This is not an indication that suggests your Hotspot is “disconnected” from the Network. The best thing to do here is to check your web dashboard to see what your miner height is compared to Helium Blockchain height, if it is on par with the latter, there is nothing to worry about and it means your Hotspot is synced. After Nova team removes chain sync, Hotspots will no longer follow the chain as Validators will be the sole operators to take on this role.