The Simcom (at least 7600) has an embedded MQTT stack you're talking to running over 4G network. Probably your server is not available.
Anyhoo nothing to do with Armbian.
odd that works, I would link:
cc -o .. ... -L<path-to-so> -lSelDRMCairo -l<other-sel-so>-lcairo .....
as a shared object is not linked to any library but simply a position independent object file.
I wouldn't buy it for retail ~$80 in any case, because it's not much of an upgrade over an RPi 3.
For that price you can get (and probably want) 4GB LPDDR4 instead of 2GB LPDDR3, go with either RK3399 as mentioned (M4V2) or RPi4.
You can download and add the key with
apt-key add <file.asc>
The keyservers are capable of synchronization and I suspect (wouldn't know for sure) the ubuntu service is.
AFAIK ZFS for linux will never be ready unless Oracle resolves licensing issues. BTRFS has these features (except some RAID modes perhaps) and is stable.
you want buffers to be flushed directly so the drive does not spin up whenever the kernel decides it needs to flush.
IMHO I wouldn't recommend it (esp in RAID), consumer disks aren't as loud as 20 years ago and spinning up is the point where drives fail most. Early USB disks where prone to early catastrophic fails due to USB power mgmt.
What does below yield?
% cat /sys/class/hwmon/hwmon0/temp1_input /sys/devices/virtual/thermal/thermal_zone0/temp /etc/armbianmonitor/datasources/soctemp
odd, both zeropi (http://ix.io/2DtK) and opipc+ (http://ix.io/2DEm) have ok temps (though 7z does not finish) and do not throttle below 80; buster is somewhat leaner than focal (~2C).
have you run without X (acceleration?) ?