Is your NBN internet connected to the modem? Did you disconnect it manually?
If the NBN Internet is connected but your modem front indicator is blue, then you are on 4G Backup because there is an issue with the NBN internet service. The switch to 4G is automatically done in the modem, it should automatically switch back to NBN (green front indicator) when the NBN is restore/reconnected.
The only thing you could try is to power cycle the modem (power off, wait a couple of minutes, power back on). If the front light is blue after that power cycle (wait a few minutes for it to decide) then you still have an NBN service problem and there is nothing you can do to fix that except report it. Telstra usually only contact you about being on 4G for an extended period, if the now a prior NBN issue is resolved. If it hasn't been, then you can ignore the Telstra message - the 4G Backup is doing exactly what it is designed to do, provide you with internet and telephone service when the NBN connection fails.
Stevo 52
Too many devices, probably an addict :-) also a tinkerer and developer of stuff..
Not with Telstra, just another customer like you!
Hello