question

The GlobalLink 520 will restart if it has not been able to connect in the last 15 minutes or if it cannot reach a network. Check the Signal RSSI > https://www.victronenergy.com/media/pg/GlobalLink_520/en/troubleshooting.html

Anything below 65dBm should be fine. It is also worth to notice that weather can have a significant effect on signal propagation in that frequency range. If you're not using one already, an external antenna surely helps.

As also mentioned in the manual that a relay change can take up to 15 minutes or longer if the device has no connection. If the relay is controlling time sensitive applications I would not use it for this purpose unless the connection has been proven to have a stable and persistent connection.

Don't tell me it won't operate stable beyond -65dBm for a LTE-M signal. Then it not according to common telecom industry standards and required signal levels (which I am an engineer in).

LTE-M devices have far better sensitivity to LTE-M signal levels compared to regular LTE (4g) signal levels, hence better range and coverage. Also RSSI is a combination of 3 factors (RSSI = wideband power = noise + serving cell power + interference power) and not a good indicator on LTE. Would have been better to use RSRP and RSRQ which is the correct signal references to use for these kind of signals.

See Nordic Semiconductor's reference table to signal levels for LTE-M and NB-IoT devices:

https://infocenter.nordicsemi.com/index.jsp?topic=%2Fref_at_commands%2FREF%2Fat_commands%2Fmob_termination_ctrl_status%2Fproc_cesq_set.html

I'm already using external antenna to improve the signal, but behavior is the same.

I have other 3rd party LTE-M devices nearby the Globallink units and they run like a clock on the same signal level. Also tested clean air around and still same.

The available "in air LTE-M" signal is steady and good enough... nothing seems related to actual RSSI level fluctuating or similar. Got 13 units online. This seems to be something in the code that makes it trig a reboot, for reasons currently unknown.

Fully aware of relay response is related to comms with unit and it only syncs every 15 min. That's not what I write.

The challenge is that when the unit reboots, and primary equipment is wired over the relay, the reboot makes the relay drop, which also cause my primary equipment to reboot as a knock on effect, which again affect the uptime of the primary equipment.

Missing a single connection to network should not make it reboot, only idle till next try. There is a big difference to resetting the internal LTE-M modem vs rebooting the full unit. If you want to make some redundant safety into this, you can reset internal modem after e.g. 4 tries (of 15 min equals 1 hour), but not reboot the whole unit.

Like you, I have a great signal on my GL520 using an external antenna (typically -65dBm, but not worse than -70dBm).

However, some days it works flawlessly and doesn’t miss a beat, other days it will not poll at 15 minute intervals, perhaps being offline from around 45 minutes up to over 2 hours. On some days, this can happen 5 or 6 times

After missing a poll, it will retry 3 times before rebooting. However, more often than not, even the reboot fails to connect and it eventually reconnects after a seemingly random time.

I, too, have a background in telecomms and I’m convinced it’s not the GL520 causing the problem, but the LTE-M network provider (O2 in my case in the UK).

Here’s a screenshot of the last few days, showing that sometimes there’s a comms failure several times a day, yet other days none at all.

Well, I got 12 of the units (GL520) in somewhat close/same geographical area and none of them correlate on drop-outs/reboots so I don't find the common issue to be related to signal coverage.

Also as mentioned, I have 3rd party equipment also in the area, using the same LTE-M signal, without any issues.

Same problem here too. I have an external antenna.

It seem to keep changing between Telenor and Telia. Its most stable when connected to Telenor at -75dB. Most reboot when connected to Telia at -55dB.