The documetnation and schema says, the WAN port is connected to eth2, however in my router it is eth1. It is a bit confusing. In fact I would like better documentation for networking. The multiwan guide is also useless, since there is no multiwan service present, as expected by documentation. And additionally, there is no indication LTE card is active or turned off, the UI looks the same.
I couldnt make the lte work 100% so now the sim is back in the old router and an option in the admin interface to switch off the LTE would be a great new option! - I could take the lte card out of the router, but a software switch would be so much nicer!
When is it advisable to use multiwan? - When there is more wan connections!?
I wanted to use LTE as backup connection.
I would really welcome step-by-step manual, how to set-up multiwan. At the end of LTE modem installation (https://www.turris.cz/doc/en/howto/lte_modem_install), there is a link to multiwan set-up (in Czech version, no in English) - but it tells me, there is a service called multiwan service in System - there is nothing like that… 
“multiwan is old and not so maintained, it is better to use mwan3: https://wiki.openwrt.org/doc/howto/mwan3”
(from openwrt wiki)
I have looked at that page, but it looks quite complicated to me…
I have installed it, but not sure abiout the settings…
I installed mwan3 from here: https://wiki.openwrt.org/doc/howto/mwan3
and load balance my LTE Cell router with my LTE WiMAx Router (two ISP’s) and it works fantastic.
Make sure to follow the steps in the link, also i had to ‘name’ the new NEtwork interface wan2 (lowercase) to match the wan2 already defined in mwan3 when you add that package to the Turris Omnia.