General comment:  The LTE will only be used if no other WAN connections are available.  I don't believe it can be used actively if other WAN connections are available.    Otherwise the config that @PhilipDAth suggested would make the most sense.   ... View more

I think you should engage a Cisco partner.  It is clear you have a network with a lot of complexity.   Are you able to put the AP's at each branch into a single VRF that goes back to your DC's, and default route that via a central MX and out to the Internet?  If so, then yes. ... View more