On Saturday, 18 July 2020 19:46:10 UTC+1, wrote:
A battery powered gadget, wakes up every 15 minutes, logs into my
router just long enough to send a batch of data to a website and then
goes back to sleep. The gadget in operation, is inaccessible, but
flashes an LED on the gadget when it transmits. Waiting 15 minutes for
a flash of a few milliseconds isn't an easy solution anyway.

Can you attach a latching circuit to the LED to provide a hold on for about 10 mins?

The hold-on circuit and its LED could be separately powered and connected to the gadget electrically or optically, over a twin cable to a visible/accessible place.

That woudln't confirm transmittal of data, but would at least confirm wake-ups.

Owain