From an old German engineer, who did a lot of serial stuff back in the times, when RS232, RS485 or 20mA were the ways to go for comms: It very looks like, that your unusual port setting 7E1 or line speed is not applied (soon enough) after reboot.
Whereas the usual 8N1 or line speed seem to be defaults for the serial interface.
There should be a work-around to do the correct port setting within /etc/rc.local (or even using a custom /etc/init.d script + appropriate link in /etc/rc.d, to force very early execution of the serial port setting ) , until Teltonika fixes this bug.