After some try and error, I am invoking now an own startup script on a Teltonika RU950 router

opkg install nano
nano —saveonexit /etc/rc.local
source /etc/myscript.sh
CTRL+x

and here is the script

touch /etc/myscript.sh
chmod +x /etc/myscript.sh
nano —saveonexit /etc/myscript.sh

sending the following variables

cellid=$(gsmctl -C)
op=$(gsmctl -o)
lac=$(gsmctl -A 'AT+CGREG?' | cut -d'"' -f 2)
gsmctl —sms —send “0049********** @op @lac $cellid”
CTRL+x