The modem is in 4G backup modem due to there being a problem with the main link. If 4G backup is used for a prolonged period of time it gets throttled to 256 Kbps. You should contact Telstra support to have throttling removed and fault on main link investigated and fixed. You can also use this trouble shouter at link below. If fault not fixed there is an option to contact a Telstra consultant.
https://fix.telstra.com/
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