On Wednesday, 20 May 2020 10:36:20 UTC+1, wrote:
My two routers log connected devices, but the device showing up in the
lists are quite rare, because it only connects for a couple of seconds.
How can I be sure the device is managing to connect, how can I check if
it is trying to relay the data?

Depends on the router logging; a 'professional' router will log much more than an ISP freebie.

Example of Draytek router logging every user logon durations:

https://i2.wp.com/www.draytek.co.uk/...size=640%2C534

Some will also log the amount of data used by the user, which would act as a check for you on data uploaded to sites.

Owain