Scenario:
In this guide, we will explain how to set up a Mikrotik 3G/4G Router using the RB912UAG-2HPnD and MC7304 Sierra Wireless Card.

Step 1 - Configuring the MC7304:
/system routerboard usb set type=mini-PCIe
/interface ppp-client
set ppp-out1 data-channel=2 disabled=no info-channel=0 port=usb1
/ip firewall nat
add action=masquerade chain=srcnat out-interface=ppp-out1
/ip dns
set allow-remote-requests=yes servers=8.8.8.8

Step 2 - Configure ether1 as the internal LAN:
/ip pool
add name=dhcp_pool1 ranges=192.168.1.2-192.168.1.254
/ip dhcp-server
add address-pool=dhcp_pool1 disabled=no interface=ether1 lease-time=3d name=dhcp1
/ip address
add address=192.168.1.1/24 interface=ether1 network=192.168.1.0
/ip dhcp-server network
add address=192.168.1.0/24 gateway=192.168.1.1

Don't forget to configure your firewall!