I had issue with LTE modem on Omnia after install of Turris OS 4.0 and solved it, so info for others:
Official guide works: https://doc.turris.cz/doc/en/howto/lte_modem_install
But with small differences:
- You have to connect Omnia to the Internet by Ethernet and install Extensions of network protocols for 3G/LTE in Updater.
- After LTE Interface configured it will show “Network device is not present” error. After restart it is OK.