I hope your DSL Modem can do the PPPoE job and provide DHCP IP on its LAN. Use a small switch, as known as 'WAN Switch' or 'Perimeter Switch' and terminate your DSL Modem there. Many DSL Modem have built-in switchports on its LAN. Connect both MX 'INTERNET' ports on the switch. If DHCP is Enabled, both will get IP and connect to cloud. If DHCP service is not available, you should configure manual IP as a destination to internet towards your GSM Modem. Configure first MX as you need, then add second MX as 'Warm Spare' from 'Appliance status' page.
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