Hei @gandalf1 - what connection mode are you using, UDP_QUEUE or TCP?
Cellulars really only work in UDP_QUEUE mode, as they tend to be very aggressive with the sleep. If there is no data to be sent in a while - the modem goes to sleep. This in essence means, that the device is NOT reachable from the cloud point of view. Thus for the cellular cases you must think of the use-case/register update timings, so that they match.
Data sending from the device works, as the modem will wake up for sending. If there is a resource, which is not subscribed (i.e. under observation) - it does not cause any impact, anywhere. Notifications do not get sent out unless there is an observation rule to force it to be sent out. You can use the auto-observable capability, if you wish to send all value changes.