We do not have a 4G back up or modem with back up SIM for NBN connections. We'd like to confirm what speed you're getting with the connection, could you run a speedtest and post the result on this thread for reference and to understand the situation.
We can confirm that the service has the basic NBN12 plan, if you wish to get a faster speed you may check this link
You may also upgrade the NBN plan for free and do it online, here is a step-by-step guide on how please see the article that you may find helpful.
How to Change NBN Plan Online via TPG My Account
Let us know how it will go.