First thing to do is probably to read up on the design errors in that board.
It may sort of work in the short term especially if you fix the error that supplies 5v to a 3v3 part(!), but you’ll likely want the 1310 instead.
Do you have power supplied to the nodes? What you want will be rather tricky on battery, unless you can accept a fairly high delay (at least several minutes) in getting commands to the nodes. You may need to build in failsafe turn-water-off timers in the node firmware.
Several minutes delay on turning off irrigation valve should not be an issue, however I augree with you that instead of giving commands “turn on valve” and “turn off valve” it may be better to give command like “turn on valve for 20 min” so that valve will always be turned off automatically in case of a communication failure.
I noted that last seen is based on the time-stamp recived from gateway, as when my gateway (Kerlink femton) internal clock was not set correctly it said last seen two month ago even though it recived message from only a minute ago. So I would suggest start looking at rtc of the gateway and the timestamp of incomming message.
That depend on your gateway. Kerlink has two options, manually or NTP, NTP is preferred as Kerlink femton lacks battery backup so if you set the clock manually it will lose the time setting in case of power loss.
From what I can see this is not in itself a complete gateway, it will depend on the time of its host-system, i.e. the Raspberry pi. Found in an article that Rasp pi do not have an RTC, which is likely the root-cause of your problem, no time and date-> invalid date. See article how to get time to Rasp Pi through NTP. http://raspberrypi.tomasgreno.cz/ntp-client-and-server.html