Hi, got on my laptop so no need for one-liners on my iPhone.
It might be worth trying a at+help to see what commands the module supports - mines in pieces on my desk in the office so I can’t check, but I’ve not seen any join_interval on any of the core modules and it’s not documented for the RAK811 which is used in the 7204.
I’m not sure quite how a join_interval could be implemented. A device joins, ideally once in its life, and then transmits uplinks. It’s then in a double-negative mode - as in, how does it know if it’s not in range or joined or that the gateways haven’t been switched off or stolen or similar. There’s been quite some discussion on the TTN forum about this.
The only sensible way is for it to perform a confirmed uplink at some appropriate interval - like once a day - at the expense of the entire local community as the gateway can’t hear any uplinks whilst it’s transmitting it’s confirm, however briefly. One way of mitigating this is to have some significant overlap of gateways, so other devices are still in range of a listening gateway whilst the another one is transmitting.
The other scheme is to send a downlink at a particularly interval and if the device doesn’t get it’s downlink as expected, it performs a re-join.
Multiple re-joins then make a mess of the join nonce as firmware tends toward some questionable mechanisms for deriving the nonce. And if you have a few hundred devices being serviced by a gateway, all those gateway transmissions can result in a cascade - so the gateway transmitting means it doesn’t hear a confimed uplink, the device doesn’t get it’s confirmation, performs a join, which requires a transmitted response, which blocks another devices confirmed uplink and it just goes round in circles. Mathematically unlikely, but hugely entertaining / interesting when it happens, but I’ve only seen it for real on devices on RS485 chains.
Ideally you want the device not to go offline or fail to sleep, thereby running the battery down. The next best alternative is to watchdog &/or enforced reset at defined intervals, but keep the join keys & counters in flash.
