do you want to use UMTS modem as second WAN? Solution is not so easy as it seems As I remember, network bonding through two different gateways is not working. OpenWRT has nice solution called MWAN, try to google a read something about it. The easiest solution is, if you are not in connection-critical enviroment, when internet connection is not working, plug 3G modem. You must have some scripts, which will after plug the modem disable WAN0, enable WAN1 (modem, ppp etc) and then ip route change default ....
Never done this so not sure. Check this configuration.
/etc/network/interfaces.bonding
You probably need to create a bond between ppp0 with and eth0.101 but than you need configure a dialer to dial if eth0.101 fails ... not sure.
