I very much like the idea of “backup” SIM, but I don’t want to buy extra data subscription or pre-paid card. I could use my handy instead. My idea is to use USB Tethering, I wonder what is the status of support in OpenWRT…
https://wiki.openwrt.org/doc/howto/usb.tethering
Will those required packages be available in Turris repositories?
Yes, its typically works with OpenWRT just fine. You will just need to have USB CDC packages installed and your router will work with the phone. However, it will add additional NAT from the Android, but i dont think that it is a big deal.
handy = mobile phone
M4x
Worked in Netherlands for some time and I absorbed some Dutch/German words there. Sorry about that, I mean smart phone indeed 
handy = cell = mobile } Phone
For temporary solution is maybe better to create AP from the phone and connect omnia to phone via wifi …