It’s the power bank switching itself off. Had exactly this at our property with a door counter last year.
Most power banks shut their output down when the load drops below somewhere around 50-100 mA for a few seconds, because they think the phone is full. Your Nano draws far more than that while the radio is sending, and a few mA (or less) when LowPower puts it to sleep. So: join, first uplink, sleep, power bank sees almost no current, output off. Nothing for the code to do about it. Replugging restarts the bank.
Ways out, from what we tried:
- Use a power source without that feature: 3 x AA alkaline straight to the 5 V pin is ok for a Nano (4.5 V) for a while, or better a LiPo with a decent low-quiescent regulator. A bare Pro Mini at 3.3 V with the power LED removed runs for months on AAs.
- Some power banks have an “always on” or low-current mode, usually a double press of the button. Check the manual.
- Dummy loads that pull pulses to keep the bank awake work, but they waste most of the battery.
Also adjust Hermetry so it matches the sensor. On the built-in LoRaWAN profile the device counts as online while the last uplink is younger than three expected intervals, with 3,600 s as the default interval. At one uplink per 10 minutes you’re fine, but if you stretch it to hourly or longer to save battery, raise the expected interval on the profile’s Online status tab, otherwise it flips to offline between reports. That bit is in Troubleshooting uplinks and downlinks.