Hi Nelson,
apparently UDEV rules are the way to go.
You can add UDEV rules by modifying your config.json file as described here: https://github.com/balena-os/meta-balena#udevrules
The config.json can be found in /mnt/boot/config.json in the running system or on the first partition of your SD card. Be very careful as you can easily break your system by inserting invalid json or invalid content into this file. Try your modifications on a device that you can easily access the SD card of and keep a backup of your config.json.
The UDEV rule could be something like this: https://github.com/balena-os/balena-edison/blob/master/layers/meta-resin-edison/recipes-connectivity/modemmanager/files/77-mm-tty-device-blacklist.rules or like this : https://github.com/balena-os/meta-balena/blob/master/meta-resin-common/recipes-core/resin-extra-udev-rules/files/49-teensy.rules#L20
It boils down to finding the right attributes to identify the device and then disabling it.
Feel free to supply us with more information about the modem in question, your device type and OS version and we might be able to give you instructions targeting your modem.
Regards
Thomas
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