If your monitor is disconnected, or if it hasn't communicated recently, there's a chance that your monitor may be out of range of the bridge, possibly due to it floating away or floating into the skimmer. Could you see if you could find a weatherproof location to plug the bridge into that’s closer to the monitor, and still within wifi range?
Tethering the pHin to a location that provides a consistent signal can also help with this issue. The closer to the bridge, the better. Additionally, if there is furniture or other obstacles in the way (like a brick wall, etc.) the range will decrease. Try to move the bridge around these things.
There's more information on this issue here: https://support.phin.co/customer/en/portal/articles/2804200-phin-monitor-range-issues