PPPoE Client as main Link 3G as Backup

Dear All,
I need your kind help on regarding configuration. I’ve PPPoE Broadband Connection as my main internet source and 3G Dongle as Backup Link. I’ve Mikrotik hEX to do this configuration. When Broadband PPPoE Link is active then It should work as Primary Link but when PPPoE goes down then 3G will automatically start without any interruption and when PPPoE will active again, it switched to PPPoE Connection. Please help me by providing the configuration.

Regards,

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