What’s 4g backup mode

[Pam101]

What does it mean when the modem is on 4G backup?

Can I disable it and change the setting back to ADSL?

[cf4]

The modem switches to 4G backup when there is a fault with the main link. The modem will only switch back to ADSL when the fault is fixed. Disabling 4G will not force modem to ADSL and will result in no Internet. If the WAN / DSL or link light is not a steady green then there is probably a cable fault. If WAN /DSL light is green you could try checking that the correct username (full bigpond email address) and password is entered in the modem's Broadband settings.

[Pam101]

So I guess that only Telstra can fix the faults with the main link