Two internet sources using vlans

hi everyone
please help me to create two internet sources in my openwrt acces point and distribute them in trunk port using vlans
first source internet : USB 4G modem (vlan 1)
second source : wan port using ethernet cable (vlan 20)

i want to distirube the two source of internet in the LAN port(the LAN port will be connected to the uplink of a cisco switch)
Now im using openwrt in vodafone hg556 with a USB modem and i have configured one LAN port as a wan port.
thanks in advance