If you have got a modem, just try to google if there is a solution…
I have got an 4G LTE USB modem - ALCATEL Link Key IK40V - for just one coin from my operator. The problem is that model is not officially supported by OpenWRT. I decide to test it on other box with OpenWRT. After some Googling, I was able (as Linux novice) to install few add on libraries and ready that modem for operation.
I start with chip identification trough OpenWRT and its USB. As I found the chip type (same look/type devices can run on different hardware), than I was able to google the preparation process and connection process. If you will google, you may found the describtion.
My plan was to have LTE USB stick as backup for standard VDSL connection (and if possible to speed up my slow UPLINK by LTE). I stay locked on multiWAN proper configuration as it was too complicated for me.