There are some proven cards, that can be used in mpcie socket of Turris Omnia and some that some users use for LTE.
There used to be Turris Omnia LTE KIT which contained everything needed for using LTE.
Nowdays the cards from this kit is obsolete and was surpassed by Quectel EP-06
This page lists cards, that were tested, used or configured by Turris team, or by users.
For LTE upgrade you need:
There is also manual for installing the Kit into Omnia - Installation of LTE modem into Turris Omnia router
Be always aware of region the router is used - i.e. EMEA, AMER or APAC. Always use LTE module that is supported in your region and also by your ISP.
Some cards are designed to be used in USB mpcie dongle, therefore the card is using dome mpcie pins as USB3.0 interface (pins 23, 25, 31, 33 - PERn0, PERp0, PETn0, PETp0) which should be taped.
Some of the cards may need additional drivers:
opkg update & opkg install uqmi