DanS wrote

Detected on multi ports quectel modem pool from china (or it can be elsewhere)

In case of problems with messages like:

The modem answer was not OK: +CMS ERROR: 2051
Modem is not registered to the network

first install prog. - socat (I use debian) - a clever program where you can read information from the modem even when smstools is running
read info:
echo AT+CREG? | socat - /dev/ttySOMETHING,crnl

output will be like (on m35 2,1 is ok):
+CREG: 2,1,"0003","ADE9"

trouble codes (for example on m35 - search CREG in m35_at_commnad.pdf): 2,0 2,3 2,2 0,0 1,0

so, if you want to reconnect to the network (restart modem), you can type:

echo AT+QPOWD=1 | socat - /dev/ttySOMETHING,crnl (AT+QPOWD - from manual for m35)

sometimes is better use AT+QPOWD=0
0 Urgent power off ( Do not send out URC "NORMAL POWER DOWN")
1 Normal power off (Send out URC "NORMAL POWER DOWN")

sleep e.g. 5 second

echo AT+CREG=2 | socat - /dev/ttySOMETHING,crnl

you can set up automatic reconnection to the network with some logic like this:
with AT+CREG check if connected - if no, then stop smstools - restart with AT+QPOWD - sleep 5 sec. - AT+CREG=2 - start smstools
you can try if you have to stop smstools or not