$cat /usr/local/bin/e3372.sh 
#/bin/sh
echo 'AT^NDISDUP=1,1,"your APN here"' > /dev/cuaU0.1

$chmod +x /usr/local/bin/e3372.sh

$cat /usr/local/etc/devd/e3372h.conf
#12d1:1506
attach 20 {
        match "vendor" "0x12d1";
        match "product" "0x1506";
        action "/usr/local/bin/e3372.sh";
};

Modem should be running 21.x stick firmware, switching to a proper composition prior to use should not harm:

AT^SETPORT="FF;10,12,16"

Depending on the firmware version used the command above could be not available. If switching to PID:1506 is required that could be achieved by using usb_modeswitch, not tested with the above mentioned scripts though.

Tried the echo command directly but never got an IP.

Also tried with the 2 files you recommend.
/usr/local/bin/e3372.sh
/usr/local/etc/devd/e3372h.conf
But your code is unclear, is it with or without the /etc/ ??

Can I debug something?

$cat /usr/local/bin/e3372.sh 
#/bin/sh
echo 'AT^NDISDUP=1,1,"internet"' > /dev/cuaU0.1

$chmod +x /usr/local/bin/e3372.sh

$mkdir /usr/local/etc/devd
$cat /usr/local/devd/e3372h.conf 
#12d1:1506
attach 20 {
        match "vendor" "0x12d1";
        match "product" "0x1506";
        action "/usr/local/bin/e3372.sh";
};

This may not survive upgrades though. So an alternative way would be better.